netfs: Fix missing xas_retry() calls in xarray iteration
Commit Message
netfslib has a number of places in which it performs iteration of an xarray
whilst being under the RCU read lock. It *should* call xas_retry() as the
first thing inside of the loop and do "continue" if it returns true in case
the xarray walker passed out a special value indicating that the walk needs
to be redone from the root[*].
Fix this by adding the missing retry checks.
[*] I wonder if this should be done inside xas_find(), xas_next_node() and
suchlike, but I'm told that's not an simple change to effect.
This can cause an oops like that below. Note the faulting address - this
is an internal value (|0x2) returned from xarray.
BUG: kernel NULL pointer dereference, address: 0000000000000402
...
RIP: 0010:netfs_rreq_unlock+0xef/0x380 [netfs]
...
Call Trace:
netfs_rreq_assess+0xa6/0x240 [netfs]
netfs_readpage+0x173/0x3b0 [netfs]
? init_wait_var_entry+0x50/0x50
filemap_read_page+0x33/0xf0
filemap_get_pages+0x2f2/0x3f0
filemap_read+0xaa/0x320
? do_filp_open+0xb2/0x150
? rmqueue+0x3be/0xe10
ceph_read_iter+0x1fe/0x680 [ceph]
? new_sync_read+0x115/0x1a0
new_sync_read+0x115/0x1a0
vfs_read+0xf3/0x180
ksys_read+0x5f/0xe0
do_syscall_64+0x38/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae
Fixes: 3d3c95046742 ("netfs: Provide readahead and readpage netfs helpers")
Reported-by: George Law <glaw@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
---
fs/netfs/buffered_read.c | 9 +++++++--
fs/netfs/io.c | 3 +++
2 files changed, 10 insertions(+), 2 deletions(-)
Comments
On Thu, Nov 03, 2022 at 09:33:28PM +0000, David Howells wrote:
> +++ b/fs/netfs/buffered_read.c
> @@ -46,10 +46,15 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
>
> rcu_read_lock();
> xas_for_each(&xas, folio, last_page) {
> - unsigned int pgpos = (folio_index(folio) - start_page) * PAGE_SIZE;
> - unsigned int pgend = pgpos + folio_size(folio);
> + unsigned int pgpos, pgend;
"unsigned int" assumes that the number of bytes isn't going to exceed 32
bits. I tend to err on the side of safety here and use size_t.
> bool pg_failed = false;
>
> + if (xas_retry(&xas, folio))
> + continue;
> +
> + pgpos = (folio_index(folio) - start_page) * PAGE_SIZE;
> + pgend = pgpos + folio_size(folio);
What happens if start_page is somewhere inside folio? Seems to me
that pgend ends up overhanging into the next folio?
Matthew Wilcox <willy@infradead.org> wrote:
> "unsigned int" assumes that the number of bytes isn't going to exceed 32
> bits. I tend to err on the side of safety here and use size_t.
Not unreasonable.
> > + pgpos = (folio_index(folio) - start_page) * PAGE_SIZE;
> > + pgend = pgpos + folio_size(folio);
>
> What happens if start_page is somewhere inside folio? Seems to me
> that pgend ends up overhanging into the next folio?
Yeah, I think my maths is dodgy. I should probably use folio_pos() and/or
offset_in_folio().
David
@@ -46,10 +46,15 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
rcu_read_lock();
xas_for_each(&xas, folio, last_page) {
- unsigned int pgpos = (folio_index(folio) - start_page) * PAGE_SIZE;
- unsigned int pgend = pgpos + folio_size(folio);
+ unsigned int pgpos, pgend;
bool pg_failed = false;
+ if (xas_retry(&xas, folio))
+ continue;
+
+ pgpos = (folio_index(folio) - start_page) * PAGE_SIZE;
+ pgend = pgpos + folio_size(folio);
+
for (;;) {
if (!subreq) {
pg_failed = true;
@@ -121,6 +121,9 @@ static void netfs_rreq_unmark_after_write(struct netfs_io_request *rreq,
XA_STATE(xas, &rreq->mapping->i_pages, subreq->start / PAGE_SIZE);
xas_for_each(&xas, folio, (subreq->start + subreq->len - 1) / PAGE_SIZE) {
+ if (xas_retry(&xas, folio))
+ continue;
+
/* We might have multiple writes from the same huge
* folio, but we mustn't unlock a folio more than once.
*/