[PATCH 0/3] Enable THP for text section of non-shmem files

[Song Liu <songliubraving@fb.com>]

This set follows up discussion at LSF/MM 2019. The motivation is to put
text section of an application in THP, and thus reduces iTLB miss rate and
improves performance. Both Facebook and Oracle showed strong interests to
this feature.

To make reviews easier, this set aims a mininal valid product. Current
version of the work does not have any changes to file system specific
code. This comes with some limitations (discussed later).

This set enables an application to "hugify" its text section by simply
running something like:

          madvise(0x600000, 0x80000, MADV_HUGEPAGE);

Before this call, the /proc/<pid>/maps looks like:

    00400000-074d0000 r-xp 00000000 00:27 2006927     app

After this call, part of the text section is splitted out and mapped to
THP:

    00400000-00425000 r-xp 00000000 00:27 2006927     app
    00600000-00e00000 r-xp 00200000 00:27 2006927     app   <<< on THP
    00e00000-074d0000 r-xp 00a00000 00:27 2006927     app

Limitations:

1. This only works for text section (vma with VM_DENYWRITE).
2. Once the application put its own pages in THP, the file is read only.
   open(file, O_WRITE) will fail with -ETXTBSY. To modify/update the file,
   it must be removed first. Here is an example case:

    root@virt-test:~/# ./app hugify
    ^C

    root@virt-test:~/# dd if=/dev/zero of=./app bs=1k count=2
    dd: failed to open './app': Text file busy

    root@virt-test:~/# cp app.backup app
    cp: cannot create regular file 'app': Text file busy

    root@virt-test:~/# rm app
    root@virt-test:~/# cp app.backup app
    root@virt-test:~/#

We gated this feature with an experimental config, READ_ONLY_THP_FOR_FS.
Once we get better support on the write path, we can remove the config and
enable it by default.

Tested cases:
1. Tested with btrfs and ext4.
2. Tested with real work application (memcache like caching service).
3. Tested with "THP aware uprobe":
   https://patchwork.kernel.org/project/linux-mm/list/?series=131339

Please share your comments and suggestions on this.

Thanks!

Song Liu (3):
  mm: check compound_head(page)->mapping in filemap_fault()
  mm,thp: stats for file backed THP
  mm,thp: add read-only THP support for (non-shmem) FS

 fs/proc/meminfo.c      |   4 ++
 include/linux/fs.h     |   8 +++
 include/linux/mmzone.h |   2 +
 mm/Kconfig             |  11 +++++
 mm/filemap.c           |   7 +--
 mm/khugepaged.c        | 107 +++++++++++++++++++++++++++++++++--------
 mm/rmap.c              |  12 +++--
 mm/vmstat.c            |   2 +
 8 files changed, 125 insertions(+), 28 deletions(-)

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2.17.1