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Subject: [v2 PATCH 0/8] Make khugepaged collapse readonly FS THP more consistent
Date:   Thu, 17 Mar 2022 16:48:19 -0700
Message-Id: <20220317234827.447799-1-shy828301@gmail.com>
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Changelog
v2: * Collected reviewed-by tags from Miaohe Lin.
    * Fixed build error for patch 4/8.

The readonly FS THP relies on khugepaged to collapse THP for suitable
vmas.  But it is kind of "random luck" for khugepaged to see the
readonly FS vmas (see report: https://lore.kernel.org/linux-mm/00f195d4-d039-3cf2-d3a1-a2c88de397a0@suse.cz/) since currently the vmas are registered to khugepaged when:
  - Anon huge pmd page fault
  - VMA merge
  - MADV_HUGEPAGE
  - Shmem mmap

If the above conditions are not met, even though khugepaged is enabled
it won't see readonly FS vmas at all.  MADV_HUGEPAGE could be specified
explicitly to tell khugepaged to collapse this area, but when khugepaged
mode is "always" it should scan suitable vmas as long as VM_NOHUGEPAGE
is not set.

So make sure readonly FS vmas are registered to khugepaged to make the
behavior more consistent.

Registering the vmas in mmap path seems more preferred from performance
point of view since page fault path is definitely hot path.


The patch 1 ~ 7 are minor bug fixes, clean up and preparation patches.
The patch 8 converts ext4 and xfs.  We may need convert more filesystems,
but I'd like to hear some comments before doing that.


Tested with khugepaged test in selftests and the testcase provided by
Vlastimil Babka in https://lore.kernel.org/lkml/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/
by commenting out MADV_HUGEPAGE call.


 b/fs/ext4/file.c                 |    4 +++
 b/fs/xfs/xfs_file.c              |    4 +++
 b/include/linux/huge_mm.h        |    9 +++++++
 b/include/linux/khugepaged.h     |   69 +++++++++++++++++++++----------------------------------------
 b/include/linux/sched/coredump.h |    3 +-
 b/kernel/fork.c                  |    4 ---
 b/mm/huge_memory.c               |   15 +++----------
 b/mm/khugepaged.c                |   71 ++++++++++++++++++++++++++++++++++++++++++++-------------------
 b/mm/shmem.c                     |   14 +++---------
 9 files changed, 102 insertions(+), 91 deletions(-)