Re: inotify on mmap writes

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 21, 2023 at 06:50:14PM -0700, Amol Dixit wrote:
> 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)?

I don't understand why you think writepages is the right place for
monitoring.  That tells you when data is leaving the page cache, not
when the file is modified.  If you want to know when the file is
modified, you need to hook into the page_mkwrite path.