Re: [PATCH] inotify: do not coalesce events with different rename cookies

[Amir Goldstein]

On Mon, Oct 26, 2026 at 1:23 AM Hungyu Lin <dennylin0707@xxxxxxxxx> wrote:
>
> inotify compares the mask, watch descriptor and name when coalescing
> adjacent unread events, but ignores their rename cookies. Distinct
> renames can therefore lose an event even with IN_ALL_EVENTS.

oops. Somehow I remembered that we never merge MOVE events.
Maybe that's even a simpler check. IDK.

>
> For example, prepare two files in an unwatched staging directory and
> rename each onto the same pathname in a watched destination directory
> before reading the events. Only the first IN_MOVED_TO event is retained,
> although the two renames have different cookies.

Thing is, the "wrong" rename TO cookie is meaningless without the FROM pair.
If the second source has a FROM, then the two TO won't merge.
Could you really reproduce this case?

> Similarly, coalescing
> distinct IN_MOVED_FROM events can discard the cookie needed to match a
> subsequent IN_MOVED_TO event.

THIS looks like a case that can be reproduced.
Can you provide an inotify LTP test for this case please.

>
> The original inotify implementation compared cookies before coalescing.
> The fsnotify conversion lost this comparison. Compare sync_cookie as well
> to preserve distinct rename events, while retaining coalescing for
> ordinary events with zero cookies.
>
> Fixes: 63c882a05416 ("inotify: reimplement inotify using fsnotify")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
> ---
>  fs/notify/inotify/inotify_fsnotify.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/notify/inotify/inotify_fsnotify.c b/fs/notify/inotify/inotify_fsnotify.c
> index 7c326ec2e8a8..6456d9c0e408 100644
> --- a/fs/notify/inotify/inotify_fsnotify.c
> +++ b/fs/notify/inotify/inotify_fsnotify.c
> @@ -40,6 +40,7 @@ static bool event_compare(struct fsnotify_event *old_fsn,
>                 return false;
>         if ((old->mask == new->mask) &&
>             (old->wd == new->wd) &&
> +           (old->sync_cookie == new->sync_cookie) &&
>             (old->name_len == new->name_len) &&
>             (!old->name_len || !strcmp(old->name, new->name)))
>                 return true;
> --
> 2.43.0
>

Practically, this means that MOVED events can never be merged,
so a slightly cheaper and simpler test would be
if ((old->mask == new->mask) && (old->mask & INOTIFY_MERGEABLE_EVENTS) && ...

Thanks,
Amir.