[v3,3/4] cachestat: implement cachestat syscall
Commit Message
Implement a new syscall that queries cache state of a file and
summarizes the number of cached pages, number of dirty pages, number of
pages marked for writeback, number of (recently) evicted pages, etc. in
a given range.
NAME
cachestat - query the page cache status of a file.
SYNOPSIS
#include <sys/mman.h>
struct cachestat {
__u64 nr_cache;
__u64 nr_dirty;
__u64 nr_writeback;
__u64 nr_evicted;
__u64 nr_recently_evicted;
};
int cachestat(unsigned int fd, off_t off, size_t len,
size_t cstat_size, struct cachestat *cstat,
unsigned int flags);
DESCRIPTION
cachestat() queries the number of cached pages, number of dirty
pages, number of pages marked for writeback, number of (recently)
evicted pages, in the bytes range given by `off` and `len`.
These values are returned in a cachestat struct, whose address is
given by the `cstat` argument.
The `off` and `len` arguments must be non-negative integers. If
`len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
0, we will query in the range from `off` to the end of the file.
`cstat_size` allows users to obtain partial results. The syscall
will copy the first `csstat_size` bytes to the specified userspace
memory. `cstat_size` must be a non-negative value that is no larger
than the current size of the cachestat struct.
The `flags` argument is unused for now, but is included for future
extensibility. User should pass 0 (i.e no flag specified).
RETURN VALUE
On success, cachestat returns 0. On error, -1 is returned, and errno
is set to indicate the error.
ERRORS
EFAULT cstat points to an invalid address.
EINVAL invalid `cstat_size` or `flags`
EBADF invalid file descriptor.
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
MAINTAINERS | 7 ++
arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/ia64/kernel/syscalls/syscall.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
include/linux/syscalls.h | 3 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 ++
init/Kconfig | 10 ++
kernel/sys_ni.c | 1 +
mm/Makefile | 1 +
mm/cachestat.c | 117 ++++++++++++++++++++
21 files changed, 165 insertions(+), 1 deletion(-)
create mode 100644 mm/cachestat.c
--
2.30.2
Comments
On Thu, Dec 8, 2022 at 12:28 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> Implement a new syscall that queries cache state of a file and
> summarizes the number of cached pages, number of dirty pages, number of
> pages marked for writeback, number of (recently) evicted pages, etc. in
> a given range.
>
> NAME
> cachestat - query the page cache status of a file.
>
> SYNOPSIS
> #include <sys/mman.h>
>
> struct cachestat {
> __u64 nr_cache;
> __u64 nr_dirty;
> __u64 nr_writeback;
> __u64 nr_evicted;
> __u64 nr_recently_evicted;
> };
>
> int cachestat(unsigned int fd, off_t off, size_t len,
> size_t cstat_size, struct cachestat *cstat,
> unsigned int flags);
>
> DESCRIPTION
> cachestat() queries the number of cached pages, number of dirty
> pages, number of pages marked for writeback, number of (recently)
> evicted pages, in the bytes range given by `off` and `len`.
>
> These values are returned in a cachestat struct, whose address is
> given by the `cstat` argument.
>
> The `off` and `len` arguments must be non-negative integers. If
> `len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
> 0, we will query in the range from `off` to the end of the file.
>
> `cstat_size` allows users to obtain partial results. The syscall
> will copy the first `csstat_size` bytes to the specified userspace
> memory. `cstat_size` must be a non-negative value that is no larger
> than the current size of the cachestat struct.
>
> The `flags` argument is unused for now, but is included for future
> extensibility. User should pass 0 (i.e no flag specified).
>
> RETURN VALUE
> On success, cachestat returns 0. On error, -1 is returned, and errno
> is set to indicate the error.
>
> ERRORS
> EFAULT cstat points to an invalid address.
>
> EINVAL invalid `cstat_size` or `flags`
>
> EBADF invalid file descriptor.
>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
> MAINTAINERS | 7 ++
> arch/alpha/kernel/syscalls/syscall.tbl | 1 +
> arch/arm/tools/syscall.tbl | 1 +
> arch/ia64/kernel/syscalls/syscall.tbl | 1 +
> arch/m68k/kernel/syscalls/syscall.tbl | 1 +
> arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
> arch/parisc/kernel/syscalls/syscall.tbl | 1 +
> arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
> arch/s390/kernel/syscalls/syscall.tbl | 1 +
> arch/sh/kernel/syscalls/syscall.tbl | 1 +
> arch/sparc/kernel/syscalls/syscall.tbl | 1 +
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
> include/linux/syscalls.h | 3 +
> include/uapi/asm-generic/unistd.h | 5 +-
> include/uapi/linux/mman.h | 9 ++
> init/Kconfig | 10 ++
> kernel/sys_ni.c | 1 +
> mm/Makefile | 1 +
> mm/cachestat.c | 117 ++++++++++++++++++++
> 21 files changed, 165 insertions(+), 1 deletion(-)
> create mode 100644 mm/cachestat.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index a198da986146..baa081a1fe52 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4552,6 +4552,13 @@ S: Supported
> F: Documentation/filesystems/caching/cachefiles.rst
> F: fs/cachefiles/
>
> +CACHESTAT: PAGE CACHE STATS FOR A FILE
> +M: Nhat Pham <nphamcs@gmail.com>
> +M: Johannes Weiner <hannes@cmpxchg.org>
> +L: linux-mm@kvack.org
> +S: Maintained
> +F: mm/cachestat.c
> +
> CADENCE MIPI-CSI2 BRIDGES
> M: Maxime Ripard <mripard@kernel.org>
> L: linux-media@vger.kernel.org
> diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> index 8ebacf37a8cf..1f13995d00d7 100644
> --- a/arch/alpha/kernel/syscalls/syscall.tbl
> +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> @@ -490,3 +490,4 @@
> 558 common process_mrelease sys_process_mrelease
> 559 common futex_waitv sys_futex_waitv
> 560 common set_mempolicy_home_node sys_ni_syscall
> +561 common cachestat sys_cachestat
> diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> index ac964612d8b0..8ebed8a13874 100644
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -464,3 +464,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
> index 72c929d9902b..f8c74ffeeefb 100644
> --- a/arch/ia64/kernel/syscalls/syscall.tbl
> +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> @@ -371,3 +371,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> index b1f3940bc298..4f504783371f 100644
> --- a/arch/m68k/kernel/syscalls/syscall.tbl
> +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> @@ -450,3 +450,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> index 820145e47350..858d22bf275c 100644
> --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> @@ -456,3 +456,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> index 8a99c998da9b..7c84a72306d1 100644
> --- a/arch/parisc/kernel/syscalls/syscall.tbl
> +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> @@ -448,3 +448,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> index 2bca64f96164..937460f0a8ec 100644
> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> @@ -530,3 +530,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 nospu set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 799147658dee..7df0329d46cb 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -453,3 +453,4 @@
> 448 common process_mrelease sys_process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat sys_cachestat
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index 2de85c977f54..97377e8c5025 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -453,3 +453,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> index 4398cc6fb68d..faa835f3c54a 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -496,3 +496,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 320480a8db4f..bc0a3c941b35 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -455,3 +455,4 @@
> 448 i386 process_mrelease sys_process_mrelease
> 449 i386 futex_waitv sys_futex_waitv
> 450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 i386 cachestat sys_cachestat
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index c84d12608cd2..8eed4cdc7965 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -372,6 +372,7 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
>
> #
> # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> index 52c94ab5c205..2b69c3c035b6 100644
> --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> @@ -421,3 +421,4 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common cachestat sys_cachestat
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index a34b0f9a9972..50f8c6999d99 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -72,6 +72,7 @@ struct open_how;
> struct mount_attr;
> struct landlock_ruleset_attr;
> enum landlock_rule_type;
> +struct cachestat;
>
> #include <linux/types.h>
> #include <linux/aio_abi.h>
> @@ -1056,6 +1057,8 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> unsigned long home_node,
> unsigned long flags);
> +asmlinkage long sys_cachestat(unsigned int fd, off_t off, size_t len,
> + size_t cstat_size, struct cachestat __user *cstat, unsigned int flags);
>
> /*
> * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 45fa180cc56a..cd639fae9086 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
> #define __NR_set_mempolicy_home_node 450
> __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
>
> +#define __NR_cachestat 451
> +__SYSCALL(__NR_cachestat, sys_cachestat)
> +
> #undef __NR_syscalls
> -#define __NR_syscalls 451
> +#define __NR_syscalls 452
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
> index f55bc680b5b0..fe03ed0b7587 100644
> --- a/include/uapi/linux/mman.h
> +++ b/include/uapi/linux/mman.h
> @@ -4,6 +4,7 @@
>
> #include <asm/mman.h>
> #include <asm-generic/hugetlb_encode.h>
> +#include <linux/types.h>
>
> #define MREMAP_MAYMOVE 1
> #define MREMAP_FIXED 2
> @@ -41,4 +42,12 @@
> #define MAP_HUGE_2GB HUGETLB_FLAG_ENCODE_2GB
> #define MAP_HUGE_16GB HUGETLB_FLAG_ENCODE_16GB
>
> +struct cachestat {
> + __u64 nr_cache;
> + __u64 nr_dirty;
> + __u64 nr_writeback;
> + __u64 nr_evicted;
> + __u64 nr_recently_evicted;
> +};
> +
> #endif /* _UAPI_LINUX_MMAN_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index 694f7c160c9c..ecc4f781dd6c 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1798,6 +1798,16 @@ config RSEQ
>
> If unsure, say Y.
>
> +config CACHESTAT
> + bool "Enable cachestat() system call" if EXPERT
> + default y
> + help
> + Enable the cachestat system call, which queries the page cache
> + statistics of a file (number of cached pages, dirty pages,
> + pages marked for writeback, (recently) evicted pages).
> +
> + If unsure say Y here.
> +
> config DEBUG_RSEQ
> default n
> bool "Enabled debugging of rseq() system call" if EXPERT
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 860b2dcf3ac4..04bfb1e4d377 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -299,6 +299,7 @@ COND_SYSCALL(set_mempolicy);
> COND_SYSCALL(migrate_pages);
> COND_SYSCALL(move_pages);
> COND_SYSCALL(set_mempolicy_home_node);
> +COND_SYSCALL(cachestat);
>
> COND_SYSCALL(perf_event_open);
> COND_SYSCALL(accept4);
> diff --git a/mm/Makefile b/mm/Makefile
> index 8e105e5b3e29..61c9c3f81cef 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -138,3 +138,4 @@ obj-$(CONFIG_IO_MAPPING) += io-mapping.o
> obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
> obj-$(CONFIG_GENERIC_IOREMAP) += ioremap.o
> obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
> +obj-$(CONFIG_CACHESTAT) += cachestat.o
> diff --git a/mm/cachestat.c b/mm/cachestat.c
> new file mode 100644
> index 000000000000..931f5e37326b
> --- /dev/null
> +++ b/mm/cachestat.c
> @@ -0,0 +1,117 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +
> +/*
> + * The cachestat() system call.
> + */
> +#include <linux/file.h>
> +#include <linux/fs.h>
> +#include <linux/syscalls.h>
> +#include <linux/shmem_fs.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
> +#include <uapi/linux/mman.h>
> +
> +#include "swap.h"
> +
> +/*
> + * The cachestat(5) system call.
> + *
> + * cachestat() returns the page cache status of a file in the
> + * bytes specified by `off` and `len`: number of cached pages,
> + * number of dirty pages, number of pages marked for writeback,
> + * number of (recently) evicted pages.
> + *
> + * `off` and `len` must be non-negative integers. If `len` > 0,
> + * the queried range is [`off`, `off` + `len`]. If `len` == 0,
> + * we will query in the range from `off` to the end of the file.
> + *
> + * `cstat_size` allows users to obtain partial results. The syscall
> + * will copy the first `csstat_size` bytes to the specified userspace
> + * memory. It also makes the cachestat struct extensible - new fields
> + * can be added in the future without breaking existing usage.
> + * `cstat_size` must be a non-negative value that is no larger than
> + * the current size of the cachestat struct.
> + *
> + * The `flags` argument is unused for now, but is included for future
> + * extensibility. User should pass 0 (i.e no flag specified).
> + *
> + * Because the status of a page can change after cachestat() checks it
> + * but before it returns to the application, the returned values may
> + * contain stale information.
> + *
> + * return values:
> + * zero - success
> + * -EFAULT - cstat points to an illegal address
> + * -EINVAL - invalid arguments
> + * -EBADF - invalid file descriptor
> + */
> +SYSCALL_DEFINE6(cachestat, unsigned int, fd, off_t, off, size_t, len,
> + size_t, cstat_size, struct cachestat __user *, cstat,
> + unsigned int, flags)
> +{
> + struct fd f;
> + struct cachestat cs;
> +
> + memset(&cs, 0, sizeof(struct cachestat));
> +
> + if (off < 0 || cstat_size > sizeof(struct cachestat) || flags != 0)
> + return -EINVAL;
> +
> + if (!access_ok(cstat, sizeof(struct cachestat)))
> + return -EFAULT;
Oops, this part should really be:
if (!access_ok(cstat, cstat_size))
I'll send a new version of patch 3/4 to fix this...
On Thu, Dec 08, 2022 at 12:28:07PM -0800, Nhat Pham wrote:
> + struct address_space *mapping = f.file->f_mapping;
> + pgoff_t first_index = off >> PAGE_SHIFT;
> + pgoff_t last_index =
> + len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
> + XA_STATE(xas, &mapping->i_pages, first_index);
> + struct folio *folio;
> +
> + rcu_read_lock();
> + xas_for_each(&xas, folio, last_index) {
> + if (xas_retry(&xas, folio) || !folio)
> + continue;
!folio can't be true. xas_for_each() terminates when folio is NULL.
> + if (xa_is_value(folio)) {
> + /* page is evicted */
> + void *shadow = (void *)folio;
> + bool workingset; /* not used */
> +
> + cs.nr_evicted += 1;
> +
> +#ifdef CONFIG_SWAP /* implies CONFIG_MMU */
> + if (shmem_mapping(mapping)) {
> + /* shmem file - in swap cache */
> + swp_entry_t swp = radix_to_swp_entry(folio);
> +
> + shadow = get_shadow_from_swap_cache(swp);
> + }
> +#endif
> + if (workingset_test_recent(shadow, true, &workingset))
> + cs.nr_recently_evicted += 1;
> +
> + continue;
> + }
> +
> + /* page is in cache */
> + cs.nr_cache += 1;
> +
> + if (folio_test_dirty(folio))
> + cs.nr_dirty += 1;
> +
> + if (folio_test_writeback(folio))
> + cs.nr_writeback += 1;
A folio may represent more than one page. That's the point of folios.
So there should be something in here which does
unsigned long nr = folio_nr_pages();
and then all these '1' should be 'nr'.
Except that you may need to adjust nr if 'first_index' > folio->index, or
'last_index' < folio->index + nr.
You should test this with XFS, AFS or erofs to be sure you're getting
results that look right.
On Mon, Dec 12, 2022 at 7:48 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Dec 08, 2022 at 12:28:07PM -0800, Nhat Pham wrote:
> > + struct address_space *mapping = f.file->f_mapping;
> > + pgoff_t first_index = off >> PAGE_SHIFT;
> > + pgoff_t last_index =
> > + len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
> > + XA_STATE(xas, &mapping->i_pages, first_index);
> > + struct folio *folio;
> > +
> > + rcu_read_lock();
> > + xas_for_each(&xas, folio, last_index) {
> > + if (xas_retry(&xas, folio) || !folio)
> > + continue;
>
> !folio can't be true. xas_for_each() terminates when folio is NULL.
Good to know - I'll remove this in the next version.
>
> > + if (xa_is_value(folio)) {
> > + /* page is evicted */
> > + void *shadow = (void *)folio;
> > + bool workingset; /* not used */
> > +
> > + cs.nr_evicted += 1;
> > +
> > +#ifdef CONFIG_SWAP /* implies CONFIG_MMU */
> > + if (shmem_mapping(mapping)) {
> > + /* shmem file - in swap cache */
> > + swp_entry_t swp = radix_to_swp_entry(folio);
> > +
> > + shadow = get_shadow_from_swap_cache(swp);
> > + }
> > +#endif
> > + if (workingset_test_recent(shadow, true, &workingset))
> > + cs.nr_recently_evicted += 1;
> > +
> > + continue;
> > + }
> > +
> > + /* page is in cache */
> > + cs.nr_cache += 1;
> > +
> > + if (folio_test_dirty(folio))
> > + cs.nr_dirty += 1;
> > +
> > + if (folio_test_writeback(folio))
> > + cs.nr_writeback += 1;
>
> A folio may represent more than one page. That's the point of folios.
> So there should be something in here which does
>
> unsigned long nr = folio_nr_pages();
>
> and then all these '1' should be 'nr'.
Yikes, that's a bug that I missed! Thanks for pointing it out.
> Except that you may need to adjust nr if 'first_index' > folio->index, or
> 'last_index' < folio->index + nr.
Sounds good!
>
> You should test this with XFS, AFS or erofs to be sure you're getting
> results that look right.
>
On Thu, Dec 8, 2022 at 9:42 PM Nhat Pham <nphamcs@gmail.com> wrote:
> Implement a new syscall that queries cache state of a file and
> summarizes the number of cached pages, number of dirty pages, number of
> pages marked for writeback, number of (recently) evicted pages, etc. in
> a given range.
>
> NAME
> cachestat - query the page cache status of a file.
>
> SYNOPSIS
> #include <sys/mman.h>
>
> struct cachestat {
> __u64 nr_cache;
> __u64 nr_dirty;
> __u64 nr_writeback;
> __u64 nr_evicted;
> __u64 nr_recently_evicted;
> };
>
> int cachestat(unsigned int fd, off_t off, size_t len,
> size_t cstat_size, struct cachestat *cstat,
> unsigned int flags);
>
> DESCRIPTION
> cachestat() queries the number of cached pages, number of dirty
> pages, number of pages marked for writeback, number of (recently)
> evicted pages, in the bytes range given by `off` and `len`.
>
> These values are returned in a cachestat struct, whose address is
> given by the `cstat` argument.
>
> The `off` and `len` arguments must be non-negative integers. If
> `len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
> 0, we will query in the range from `off` to the end of the file.
>
> `cstat_size` allows users to obtain partial results. The syscall
> will copy the first `csstat_size` bytes to the specified userspace
> memory. `cstat_size` must be a non-negative value that is no larger
> than the current size of the cachestat struct.
>
> The `flags` argument is unused for now, but is included for future
> extensibility. User should pass 0 (i.e no flag specified).
>
> RETURN VALUE
> On success, cachestat returns 0. On error, -1 is returned, and errno
> is set to indicate the error.
>
> ERRORS
> EFAULT cstat points to an invalid address.
>
> EINVAL invalid `cstat_size` or `flags`
>
> EBADF invalid file descriptor.
>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> arch/m68k/kernel/syscalls/syscall.tbl | 1 +
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> [m68k]
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
@@ -4552,6 +4552,13 @@ S: Supported
F: Documentation/filesystems/caching/cachefiles.rst
F: fs/cachefiles/
+CACHESTAT: PAGE CACHE STATS FOR A FILE
+M: Nhat Pham <nphamcs@gmail.com>
+M: Johannes Weiner <hannes@cmpxchg.org>
+L: linux-mm@kvack.org
+S: Maintained
+F: mm/cachestat.c
+
CADENCE MIPI-CSI2 BRIDGES
M: Maxime Ripard <mripard@kernel.org>
L: linux-media@vger.kernel.org
@@ -490,3 +490,4 @@
558 common process_mrelease sys_process_mrelease
559 common futex_waitv sys_futex_waitv
560 common set_mempolicy_home_node sys_ni_syscall
+561 common cachestat sys_cachestat
@@ -464,3 +464,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -371,3 +371,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -450,3 +450,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -456,3 +456,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -448,3 +448,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -530,3 +530,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 nospu set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -453,3 +453,4 @@
448 common process_mrelease sys_process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat sys_cachestat
@@ -453,3 +453,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -496,3 +496,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -455,3 +455,4 @@
448 i386 process_mrelease sys_process_mrelease
449 i386 futex_waitv sys_futex_waitv
450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 i386 cachestat sys_cachestat
@@ -372,6 +372,7 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
#
# Due to a historical design error, certain syscalls are numbered differently
@@ -421,3 +421,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
@@ -72,6 +72,7 @@ struct open_how;
struct mount_attr;
struct landlock_ruleset_attr;
enum landlock_rule_type;
+struct cachestat;
#include <linux/types.h>
#include <linux/aio_abi.h>
@@ -1056,6 +1057,8 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
unsigned long home_node,
unsigned long flags);
+asmlinkage long sys_cachestat(unsigned int fd, off_t off, size_t len,
+ size_t cstat_size, struct cachestat __user *cstat, unsigned int flags);
/*
* Architecture-specific system calls
@@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
#define __NR_set_mempolicy_home_node 450
__SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
+#define __NR_cachestat 451
+__SYSCALL(__NR_cachestat, sys_cachestat)
+
#undef __NR_syscalls
-#define __NR_syscalls 451
+#define __NR_syscalls 452
/*
* 32 bit systems traditionally used different
@@ -4,6 +4,7 @@
#include <asm/mman.h>
#include <asm-generic/hugetlb_encode.h>
+#include <linux/types.h>
#define MREMAP_MAYMOVE 1
#define MREMAP_FIXED 2
@@ -41,4 +42,12 @@
#define MAP_HUGE_2GB HUGETLB_FLAG_ENCODE_2GB
#define MAP_HUGE_16GB HUGETLB_FLAG_ENCODE_16GB
+struct cachestat {
+ __u64 nr_cache;
+ __u64 nr_dirty;
+ __u64 nr_writeback;
+ __u64 nr_evicted;
+ __u64 nr_recently_evicted;
+};
+
#endif /* _UAPI_LINUX_MMAN_H */
@@ -1798,6 +1798,16 @@ config RSEQ
If unsure, say Y.
+config CACHESTAT
+ bool "Enable cachestat() system call" if EXPERT
+ default y
+ help
+ Enable the cachestat system call, which queries the page cache
+ statistics of a file (number of cached pages, dirty pages,
+ pages marked for writeback, (recently) evicted pages).
+
+ If unsure say Y here.
+
config DEBUG_RSEQ
default n
bool "Enabled debugging of rseq() system call" if EXPERT
@@ -299,6 +299,7 @@ COND_SYSCALL(set_mempolicy);
COND_SYSCALL(migrate_pages);
COND_SYSCALL(move_pages);
COND_SYSCALL(set_mempolicy_home_node);
+COND_SYSCALL(cachestat);
COND_SYSCALL(perf_event_open);
COND_SYSCALL(accept4);
@@ -138,3 +138,4 @@ obj-$(CONFIG_IO_MAPPING) += io-mapping.o
obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
obj-$(CONFIG_GENERIC_IOREMAP) += ioremap.o
obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
+obj-$(CONFIG_CACHESTAT) += cachestat.o
new file mode 100644
@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/*
+ * The cachestat() system call.
+ */
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/syscalls.h>
+#include <linux/shmem_fs.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
+#include <uapi/linux/mman.h>
+
+#include "swap.h"
+
+/*
+ * The cachestat(5) system call.
+ *
+ * cachestat() returns the page cache status of a file in the
+ * bytes specified by `off` and `len`: number of cached pages,
+ * number of dirty pages, number of pages marked for writeback,
+ * number of (recently) evicted pages.
+ *
+ * `off` and `len` must be non-negative integers. If `len` > 0,
+ * the queried range is [`off`, `off` + `len`]. If `len` == 0,
+ * we will query in the range from `off` to the end of the file.
+ *
+ * `cstat_size` allows users to obtain partial results. The syscall
+ * will copy the first `csstat_size` bytes to the specified userspace
+ * memory. It also makes the cachestat struct extensible - new fields
+ * can be added in the future without breaking existing usage.
+ * `cstat_size` must be a non-negative value that is no larger than
+ * the current size of the cachestat struct.
+ *
+ * The `flags` argument is unused for now, but is included for future
+ * extensibility. User should pass 0 (i.e no flag specified).
+ *
+ * Because the status of a page can change after cachestat() checks it
+ * but before it returns to the application, the returned values may
+ * contain stale information.
+ *
+ * return values:
+ * zero - success
+ * -EFAULT - cstat points to an illegal address
+ * -EINVAL - invalid arguments
+ * -EBADF - invalid file descriptor
+ */
+SYSCALL_DEFINE6(cachestat, unsigned int, fd, off_t, off, size_t, len,
+ size_t, cstat_size, struct cachestat __user *, cstat,
+ unsigned int, flags)
+{
+ struct fd f;
+ struct cachestat cs;
+
+ memset(&cs, 0, sizeof(struct cachestat));
+
+ if (off < 0 || cstat_size > sizeof(struct cachestat) || flags != 0)
+ return -EINVAL;
+
+ if (!access_ok(cstat, sizeof(struct cachestat)))
+ return -EFAULT;
+
+ f = fdget(fd);
+ if (f.file) {
+ struct address_space *mapping = f.file->f_mapping;
+ pgoff_t first_index = off >> PAGE_SHIFT;
+ pgoff_t last_index =
+ len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
+ XA_STATE(xas, &mapping->i_pages, first_index);
+ struct folio *folio;
+
+ rcu_read_lock();
+ xas_for_each(&xas, folio, last_index) {
+ if (xas_retry(&xas, folio) || !folio)
+ continue;
+
+ if (xa_is_value(folio)) {
+ /* page is evicted */
+ void *shadow = (void *)folio;
+ bool workingset; /* not used */
+
+ cs.nr_evicted += 1;
+
+#ifdef CONFIG_SWAP /* implies CONFIG_MMU */
+ if (shmem_mapping(mapping)) {
+ /* shmem file - in swap cache */
+ swp_entry_t swp = radix_to_swp_entry(folio);
+
+ shadow = get_shadow_from_swap_cache(swp);
+ }
+#endif
+ if (workingset_test_recent(shadow, true, &workingset))
+ cs.nr_recently_evicted += 1;
+
+ continue;
+ }
+
+ /* page is in cache */
+ cs.nr_cache += 1;
+
+ if (folio_test_dirty(folio))
+ cs.nr_dirty += 1;
+
+ if (folio_test_writeback(folio))
+ cs.nr_writeback += 1;
+ }
+ rcu_read_unlock();
+ fdput(f);
+
+ if (copy_to_user(cstat, &cs, cstat_size))
+ return -EFAULT;
+
+ return 0;
+ }
+
+ return -EBADF;
+}