mbox series

[0/2] change ->index to PAGE_SIZE for hugetlb pages

Message ID 20230609194947.37196-1-sidhartha.kumar@oracle.com
Headers
Series change ->index to PAGE_SIZE for hugetlb pages |

Message

Sidhartha Kumar June 9, 2023, 7:49 p.m. UTC 
  ========================== OVERVIEW ========================================
This patchset attempts to implement a listed filemap TODO which is
changing hugetlb folios to have ->index in PAGE_SIZE. This simplifies many
functions within filemap.c as they have to special case hugetlb pages.
From the RFC v1[1], Mike pointed out that hugetlb will still have to maintain
a huge page sized index as it is used for the reservation map and the hash
function for the hugetlb mutex table.

This patchset adds new wrappers for hugetlb code to to interact with the
page cache. These wrappers calculate a linear page index as this is now
what the page cache expects for hugetlb pages.

From the discussion on HGM for hugetlb[3], there is a want to remove hugetlb
special casing throughout the core mm code. This series accomplishes
a part of this by shifting complexity from filemap.c to hugetlb.c. There
are still checks for hugetlb within the filemap code as cgroup accounting
and hugetlb accounting are special cased as well. 

=========================== PERFORMANCE =====================================
The time spent in hugetlb_add_to_page_cache() and __filemap_get_folio() was
tracked while running the LTP hugetlb testcases. The units are in nanoseconds.

6.4.0-rc5
@hugetlb_add_to_page_cache:                                                                                                                                                                                                     
[512, 1K)           7518 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|                                                                                                                                   
[1K, 2K)             158 |@                                                   |                                                                                                                                   
[2K, 4K)              30 |                                                    |                                                                                                                                   
[4K, 8K)               6 |                                                    |                                                                                                                                   
[8K, 16K)              9 |                                                    | 


6.4.0-rc5 + this patch series
@hugetlb_add_to_page_cache:                                                                                                                                                                                                     
[512, 1K)           6400 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|                                                                                                                                   
[1K, 2K)            1230 |@@@@@@@@@                                           |                                                                                                                                   
[2K, 4K)              55 |                                                    |                                                                                                                                   
[4K, 8K)              16 |                                                    |                                                                                                                                   
[8K, 16K)             19 |                                                    |                                                                                                                                   
[16K, 32K)             1 |     

6.4.0-rc5
@__filemap_get_folio:                                                                                                                                                                                                    
[256, 512)         11292 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|                                                                                                                                   
[512, 1K)           4615 |@@@@@@@@@@@@@@@@@@@@@                               |                                                                                                                                   
[1K, 2K)             960 |@@@@                                                |                                                                                                                                   
[2K, 4K)             188 |                                                    |                                                                                                                                   
[4K, 8K)              68 |                                                    |                                                                                                                                   
[8K, 16K)             14 |                                                    |                                                                                                                                   
[16K, 32K)             4 |                                                    |                                                                                                                                                                                                                                                                
[2G, 4G)               4 |                                                    |

6.4.0-rc5 + this patch series
@__filemap_get_folio:                                                                                                                                                                                                    
[128, 256)             4 |                                                    |                                                                                                                                   
[256, 512)         11068 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|                                                                                                                                   
[512, 1K)           3334 |@@@@@@@@@@@@@@@                                     |                                                                                                                                   
[1K, 2K)             559 |@@                                                  |                                                                                                                                   
[2K, 4K)             112 |                                                    |                                                                                                                                   
[4K, 8K)              42 |                                                    |                                                                                                                                   
[8K, 16K)              7 |                                                    |                                                                                                                                   
[16K, 32K)             2 |                                                    |                                                                                                                                   
[2G, 4G)               3 |                                                    |

This performance characteristics are similar after the patch with slightly more
latency in hugetlb_add_to_page_cache() seen by the increase of events within the
[1K, 2K) bucket. Please let me know if I should test more functions for performance
impact.

=========================== TESTING ==========================================
This series passes the LTP hugetlb test cases.

RFC v2[2]-> v1:
  -cleanup code style

RFC v1 -> v2
  -change direction of series to maintain both huge and base page size index
   rather than try to get rid of all references to a huge page sized index.

rebased on 06/09/2023 mm-unstable

[1]:https://lore.kernel.org/lkml/20230425012721.GA6696@monkey/T/
[2]:https://lore.kernel.org/linux-mm/20230519220142.212051-1-sidhartha.kumar@oracle.com/T/
[3]:https://lore.kernel.org/linux-mm/a49e7ec8-735d-5a81-1744-cb887389a559@google.com/T/#r78a847a834671a9a7095c9458d9a1737015e7d0b

Sidhartha Kumar (2):
  mm/filemap: remove hugetlb special casing in filemap.c
  mm/hugetlb: add wrapper functions for interactions with page cache

 fs/hugetlbfs/inode.c    | 14 +++++++-------
 include/linux/hugetlb.h | 21 +++++++++++++++++++--
 include/linux/pagemap.h |  8 --------
 mm/filemap.c            | 36 +++++++++++-------------------------
 mm/hugetlb.c            | 22 +++++++++++++---------
 5 files changed, 50 insertions(+), 51 deletions(-)