Re: [PATCH] ovl: add RWF_NONITIFY flag to skip wrong duplicate fanotify event

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, Apr 23, 2019 at 3:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Tue, Apr 23, 2019 at 4:41 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Tue, Apr 23, 2019 at 2:44 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Tue, Apr 23, 2019 at 2:40 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> > > >
> > > > On Tue, Apr 23, 2019 at 1:00 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > >
> > > > > On Tue, Apr 23, 2019 at 9:51 AM Murphy Zhou <jencce.kernel@gmail.com> wrote:
> > > > > >
> > > > > > Overlays ovl_iter_write calls vfs_iter_write to write on real file,
> > > > > > in which calls fsnotify_modify on this change, however vfs_write also
> > > > > > calls fsnotify_modify after ovl_iter_write. The first notification
> > > > > > sent by vfs_iter_write grabs marks from upper inode and overlay mnt,
> > > > > > because of its fake path. The second one sent by vfs_write grabs marks
> > > > > > from ovl inode and ovl mnt.
> > > > > >
> > > > > > LTP fanotify06 add modify mark for mnt point, then add ignore modify
> > > > > > mask on testfile, then truncate and write the file. Because the ignore
> > > > > > mask is marked on ovl inode, not the upper inode, the first event is not
> > > > > > masked like the second one. So we get a modification event even with a
> > > > > > mask on the file.
> > > > >
> > > > > Care to extend fanotify06 in a similar manner to the way readahead02
> > > > > was extended to test overlay test case regardless of the base LTP filesystem?
> > > > >
> > > > > >
> > > > > > Proposing fixing this by add a new RWF flag to skip fsnotify on this IO.
> > > > > > vfs_iter_write used by ovl can use this flag to skip one duplicate event.
> > > > > >
> > > > >
> > > > > This fix is wrong for several reasons:
> > > > > - It exports RWF_NONOTIFY to uapi
> > > > > - It will cause no events at all when overlay writes to file even when user
> > > > > requested events on upper inode
> > > > >
> > > > > Please try attached patch.
> > > >
> > > > Would be nice, but until mmap stops using realfile this isn't a good solution.
> > > >
> > >
> > > Sigh! I figured there was a catch...
> > > Will it be ok if fake path used a cloned private mount of overlay mount?
> >
> > So the reason we have the fake path is e.g. /proc/$$/maps.  That can
> > only work with the original mount, AFAICS.
> >
> > We could have one realfile for regular I/O and a separate one for mmap
> > but that would increase complexity as well as resource use, so I'm not
> > quite sure if that's the right solution.
> >
>
> I see. Well what we could do is set a file flag for fake path, so at least
> fsnotify will be able to ignore f_path->mnt when calculating ignore mask.

FMODE_NONOTIFY_MNT: like FMODE_NONOTIFY, but only affect mount marks?

Thanks,
Miklos