On Wed, Mar 22, 2023 at 12:16 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > On Wed, Mar 22, 2023 at 4:13 AM Amol Dixit <amoldd@xxxxxxxxx> wrote: > > > > 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. > > > > What is the use case? > Would it be sufficient if you had an OPEN_WRITE event? > or if OPEN event had the O_ flags as an extra info to the event? > I have a patch for the above and I personally find this information > missing from OPEN events. > > Are you trying to monitor mmap() calls? write to an mmaped area? > because writepages() will get you neither of these. OPEN events are not useful to track file modifications in real time, although I can do see the usefulness of OPEN_WRITE events to track files that can change. I am trying to track writes to mmaped area (as these are not notified using inotify events). I wanted to ask the community of the feasibility and usefulness of this. I had some design ideas of tracking writes (using jbd commit callbacks for instance) in the kernel, but to make it generic sprucing up the inotify interface is a much better approach. Hope that provides some context. Thanks, Amol
