linux-ext4 - Re: [PATCH v2 01/14] fsnotify: Don't call insert hook for overflow events

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Message-ID: <CAOQ4uxh-4dRSWQZr0Y7WrVmQc1ZLo=WXhmqD3DDMrWn0CgSC-A@mail.gmail.com>
Date:   Wed, 16 Jun 2021 10:49:10 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     kernel@...labora.com, "Darrick J. Wong" <djwong@...nel.org>,
        Theodore Tso <tytso@....edu>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
        David Howells <dhowells@...hat.com>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2 01/14] fsnotify: Don't call insert hook for overflow events

On Wed, Jun 16, 2021 at 2:56 AM Gabriel Krisman Bertazi
<krisman@...labora.com> wrote:
>
> Overflow events are not mergeable, so they are not hashed_events.  But,
> when failing inside fsnotify_add_event, for lack of space,
> fsnotify_add_event() still calls the insert hook, which adds the
> overflow event to the merge list.
>
> Avoid calling the insert hook when adding an overflow event.
>
> Fixes: 94e00d28a680 ("fsnotify: use hash table for faster events merge")
> Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
> ---
>  fs/notify/notification.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/fs/notify/notification.c b/fs/notify/notification.c
> index 32f45543b9c6..033294669e07 100644
> --- a/fs/notify/notification.c
> +++ b/fs/notify/notification.c
> @@ -106,6 +106,11 @@ int fsnotify_add_event(struct fsnotify_group *group,
>                         return ret;
>                 }
>                 event = group->overflow_event;
> +               /*
> +                * Since overflow events are not mergeable, don't insert
> +                * them in the merge hash.
> +                */
> +               insert = NULL;
>                 goto queue;
>         }
>

Hmm, the fix looks correct, but a bit fragile.
While it makes sense that @insert is the counterpart of @merge
there is nothing in the API that mandates it.

Therefore, it would be more robust IMO to add a check
fanotify_is_hashed_event(mask) in fanotify_insert_event()
to match fanotify_is_hashed_event(mask)  in get_one_event().

If we do that, we can also simplify:

--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -779,8 +779,7 @@ static int fanotify_handle_event(struct
fsnotify_group *group, u32 mask,

        fsn_event = &event->fse;
        ret = fsnotify_add_event(group, fsn_event, fanotify_merge,
-                                fanotify_is_hashed_event(mask) ?
-                                fanotify_insert_event : NULL);
+                                fanotify_insert_event);


Thanks,
Amir.