This patch adds a new mapping flag AS_NO_DATA_INTEGRITY which signifies that a
mapping does not have data integrity guarantees, and skips waiting on writeout
for these mappings in wait_sb_inodes(), as these mappings cannot guarantee
that data is persistently stored. This patch sets this flag on fuse mappings.

This fixes the userspace regression reported by Athul and J. upstream in
[1][2] where if there is a bug in a fuse server that causes the server to
never complete writeback, it will make wait_sb_inodes() wait forever.

Thanks,
Joanne

[1] https://lore.kernel.org/regressions/CAJnrk1ZjQ8W8NzojsvJPRXiv9TuYPNdj8Ye7=Cgkj=iV_i8EaA@mail.gmail.com/T/#t (local)
[2] https://lore.kernel.org/linux-fsdevel/aT7JRqhUvZvfUQlV@eldamar.lan/ (local)

Changelog:
v2: https://lore.kernel.org/linux-fsdevel/20251215030043.1431306-1-joannelkoong@gmail.com/ (local)
* Add comments to commit message (David) and to wait_sb_inodes() (Andrew)
* Add Bernd's Reviewed-by and J's Tested-by

v1: https://lore.kernel.org/linux-mm/20251120184211.2379439-1-joannelkoong@gmail.com/ (local)
* Change AS_WRITEBACK_MAY_HANG to AS_NO_DATA_INTEGRITY and keep
  AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM as is.

Joanne Koong (1):
  fs/writeback: skip AS_NO_DATA_INTEGRITY mappings in wait_sb_inodes()

 fs/fs-writeback.c       |  7 ++++++-
 fs/fuse/file.c          |  4 +++-
 include/linux/pagemap.h | 11 +++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

-- 
2.47.3