[PATCH v2 0/1] iomap: don't mark folio uptodate if read IO has bytes pending

[Joanne Koong <joannelkoong@gmail.com>]

This is a fix for this scenario:

->read_folio() gets called on a folio size that is 16k while the file is 4k:
  a) ifs->read_bytes_pending gets initialized to 16k
  b) ->read_folio_range() is called for the 4k read
  c) the 4k read succeeds, ifs->read_bytes_pending is now 12k and the
0 to 4k range is marked uptodate
  d) the post-eof blocks are zeroed and marked uptodate in the call to
iomap_set_range_uptodate()
  e) iomap_set_range_uptodate() sees all the ranges are marked
uptodate and it marks the folio uptodate
  f) iomap_read_end() gets called to subtract the 12k from
ifs->read_bytes_pending. it too sees all the ranges are marked
uptodate and marks the folio uptodate using XOR
  g) the XOR call clears the uptodate flag on the folio

The same situation can occur if the last range read for the folio is done as
an inline read and all the previous ranges have already completed by the time
the inline read completes.

For more context, the full discussion can be found in [1]. There was a
discussion about alternative approaches in that thread, but they had more
complications.

There is another discussion in v1 [2] about consolidating the read paths.
Until that is resolved, this patch fixes the issue.

Thanks,
Joanne

[1] https://lore.kernel.org/linux-fsdevel/CAJnrk1Z9za5w4FoJqTGx50zR2haHHaoot1KJViQyEHJQq4=34w@mail.gmail.com/#t
[2] https://lore.kernel.org/linux-fsdevel/20260219003911.344478-1-joannelkoong@gmail.com/T/#u

Changelog
---------
v1: https://lore.kernel.org/linux-fsdevel/20260219003911.344478-1-joannelkoong@gmail.com/T/#u
Changes made to v1:
* Add Darrick's reviewed-by, cc stable@, add link to discussion to commit
  message (Darrick)

Joanne Koong (1):
  iomap: don't mark folio uptodate if read IO has bytes pending

 fs/iomap/buffered-io.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

-- 
2.47.3