The tmpfs has already supported the PMD-sized large folios, but the tmpfs
read operation still performs copying at the PAGE SIZE granularity, which
is not perfect. This patch changes to copy data at the folio granularity,
which can improve the read performance.

Use 'fio bs=64k' to read a 1G tmpfs file populated with 2M THPs, and I can
see about 20% performance improvement, and no regression with bs=4k. I
also did some functional test with the xfstests suite, and I did not find
any regressions with the following xfstests config.
  FSTYP=tmpfs
  export TEST_DIR=/mnt/tempfs_mnt
  export TEST_DEV=/mnt/tempfs_mnt
  export SCRATCH_MNT=/mnt/scratchdir
  export SCRATCH_DEV=/mnt/scratchdir

Changes from v1:
 - Move index calculation to the appropriate place, per Kefeng.
 - Fallback to page copy if large folio has poisoned subpages, suggested
   by Matthew and Yang.

Baolin Wang (2):
  mm: shmem: update iocb->ki_pos directly to simplify tmpfs read logic
  mm: shmem: improve the tmpfs large folio read performance

 mm/shmem.c | 65 +++++++++++++++++++++++++++---------------------------
 1 file changed, 33 insertions(+), 32 deletions(-)

-- 
2.39.3