Re: [PATCH v3 04/15] fanotify: Split superblock marks out to a new cache

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Jul 7, 2021 at 11:13 PM Jan Kara <jack@suse.cz> wrote:
>
> On Tue 29-06-21 15:10:24, Gabriel Krisman Bertazi wrote:
> > FAN_ERROR will require an error structure to be stored per mark.  But,
> > since FAN_ERROR doesn't apply to inode/mount marks, it should suffice to
> > only expose this information for superblock marks. Therefore, wrap this
> > kind of marks into a container and plumb it for the future.
> >
> > Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
>
> ...
>
> > -static void fanotify_free_mark(struct fsnotify_mark *fsn_mark)
> > +static void fanotify_free_mark(struct fsnotify_mark *mark)
> >  {
> > -     kmem_cache_free(fanotify_mark_cache, fsn_mark);
> > +     if (mark->flags & FSNOTIFY_MARK_FLAG_SB) {
> > +             struct fanotify_sb_mark *fa_mark = FANOTIFY_SB_MARK(mark);
> > +
> > +             kmem_cache_free(fanotify_sb_mark_cache, fa_mark);
> > +     } else {
> > +             kmem_cache_free(fanotify_mark_cache, mark);
> > +     }
> >  }
>
> Frankly, I find using mark->flags for fanotify internal distinction of mark
> type somewhat ugly. Even more so because fsnotify_put_mark() could infer
> the mark type information from mark->conn->type and pass it to the freeing
> function. But the passing would be somewhat non-trivial so probably we can
> leave that for some other day. But the fact that FSNOTIFY_MARK_FLAG_SB is
> set inside fsnotify-backend specific code is a landmine waiting just for
> another backend to start supporting sb marks, not set
> FSNOTIFY_MARK_FLAG_SB, and some generic code depend on checking
> FSNOTIFY_MARK_FLAG_SB instead of mark->conn->type.
>
> So I see two sensible solutions:
>
> a) Just admit this is fanotify private flag, carve out some flags from
> mark->flags as backend private and have FANOTIFY_MARK_FLAG_SB in that space
> (e.g. look how include/linux/buffer_head.h has flags upto BH_PrivateStart,
> then e.g. include/linux/jbd2.h starts its flags from BH_PrivateStart
> further).
>
> b) Make a rule that mark connector type is also stored in mark->flags. We
> have plenty of space there so why not. Then fsnotify_add_mark_locked() has
> to store type into the flags.
>
> Pick your poison :)

I find option a) more flexible for future expansion.
This way fanotify could potentially allocate "normal" sb marks
(e.g. if not FAN_REPORT_FID) and "private" sb marks otherwise
(not that I recommend it - this was just an example)

Also, it is far less fsnotify code changes:

 #define FSNOTIFY_MARK_FLAG_ATTACHED            0x04
+/* Backend private flags */
+#define FSNOTIFY_MARK_FLAG_PRIVATE(x)   (0x100 + (x))

[...]

+#define FANOTIFY_MARK_FLAG_SB FSNOTIFY_MARK_FLAG_PRIVATE(0x01)
+
+struct fanotify_sb_mark {
+       struct fsnotify_mark fsn_mark;
+};
+

Thanks,
Amir.