inotify on mmap writes

[Amol Dixit <amoldd@gmail.com>]

Hello,
Apologies if this has been discussed or clarified in the past.

The lack of file modification notification events (inotify, fanotify)
for mmap() regions is a big hole to anybody watching file changes from
userspace. I can imagine atleast 2 reasons why that support may be
lacking, perhaps there are more:

1. mmap() writeback is async (unless msync/fsync triggered) driven by
file IO and page cache writeback mechanims, unlike write system calls
that get funneled via the vfs layer, whih is a convenient common place
to issue notifications. Now mm code would have to find a common ground
with filesystem/vfs, which is messy.

2. writepages, being an address-space op is treated by each file
system independently. If mm did not want to get involved, onus would
be on each filesystem to make their .writepages handlers notification
aware. This is probably also considered not worth the trouble.

So my question is, notwithstanding minor hurdles (like lost events,
hardlinks etc.), would the community like to extend inotify support
for mmap'ed writes to files? Under configs options, would a fix on a
per filesystem basis be an acceptable solution (I can start with say
ext4 writepages linking back to inode/dentry and firing a
notification)?

Eventually we will have larger support across the board and
inotify/fanotify can be a reliable tracking mechanism for
modifications to files.

Thank you,
Amol