[v5,0/2] mm,thp: Add filemap_huge_fault() for THP

Message

William Kucharski Sept. 2, 2019, 9:23 a.m. UTC

This set of patches is the first step towards a mechanism for automatically
mapping read-only text areas of appropriate size and alignment to THPs
whenever possible.

For now, the central routine, filemap_huge_fault(), amd various support
routines are only included if the experimental kernel configuration option

        RO_EXEC_FILEMAP_HUGE_FAULT_THP

is enabled.

This is because filemap_huge_fault() is dependent upon the
address_space_operations vector readpage() pointing to a routine that will
read and fill an entire large page at a time without poulluting the page
cache with PAGESIZE entries for the large page being mapped or performing
readahead that would pollute the page cache entries for succeeding large
pages. Unfortunately, there is no good way to determine how many bytes
were read by readpage(). At present, if filemap_huge_fault() were to call
a conventional readpage() routine, it would only fill the first PAGESIZE
bytes of the large page, which is definitely NOT the desired behavior.

However, by making the code available now it is hoped that filesystem
maintainers who have pledged to provide such a mechanism will do so more
rapidly.

The first part of the patch adds an order field to __page_cache_alloc(),
allowing callers to directly request page cache pages of various sizes.
This code was provided by Matthew Wilcox.

The second part of the patch implements the filemap_huge_fault() mechanism
as described above.

As this code is only run when the experimental config option is set,
there are some issues that need to be resolved but this is a good step
step that will enable further developemt.

Changes since v4:
1. More code review comments addressed, fixed bug in rcu logic
2. Add code to delete hugepage from page cache upon failure within
   filemap_huge_fault
3. Remove improperly crafted VM_BUG_ON when removing a THP from the
   page cache
4. Fixed mapping count issue

Changes since v3:
1. Multiple code review comments addressed
2. filemap_huge_fault() now does rcu locking when possible
3. filemap_huge_fault() now properly adds the THP to the page cache before
   calling readpage()

Changes since v2:
1. FGP changes were pulled out to enable submission as an independent
   patch
2. Inadvertent tab spacing and comment changes were reverted

Changes since v1:
1. Fix improperly generated patch for v1 PATCH 1/2

Matthew Wilcox (1):
  Add an 'order' argument to __page_cache_alloc() and
    do_read_cache_page(). Ensure the allocated pages are compound pages.

William Kucharski (1):
  Add filemap_huge_fault() to attempt to satisfy page faults on
    memory-mapped read-only text pages using THP when possible.

 fs/afs/dir.c            |   2 +-
 fs/btrfs/compression.c  |   2 +-
 fs/cachefiles/rdwr.c    |   4 +-
 fs/ceph/addr.c          |   2 +-
 fs/ceph/file.c          |   2 +-
 include/linux/mm.h      |   2 +
 include/linux/pagemap.h |  10 +-
 mm/Kconfig              |  15 ++
 mm/filemap.c            | 418 ++++++++++++++++++++++++++++++++++++++--
 mm/huge_memory.c        |   3 +
 mm/mmap.c               |  39 +++-
 mm/readahead.c          |   2 +-
 mm/rmap.c               |   4 +-
 mm/vmscan.c             |   2 +-
 net/ceph/pagelist.c     |   4 +-
 net/ceph/pagevec.c      |   2 +-
 16 files changed, 473 insertions(+), 40 deletions(-)