fsnotify: optimize FS_MODIFY events with no ignored masks

fsnotify() treats FS_MODIFY events specially - it does not skip them
even if the FS_MODIFY event does not apear in the object's fsnotify
mask. This is because send_to_group() checks if FS_MODIFY needs to
clear ignored mask of marks.

The common case is that an object does not have any mark with ignored
mask and in particular, that it does not have a mark with ignored mask
and without the FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY flag.

Set FS_MODIFY in object's fsnotify mask during fsnotify_recalc_mask()
if object has a mark with an ignored mask and without the
FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY flag and remove the special
treatment of FS_MODIFY in fsnotify(), so that FS_MODIFY events could
be optimized in the common case.

Call fsnotify_recalc_mask() from fanotify after adding or removing an
ignored mask from a mark without FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY
or when adding the FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY flag to a mark
with ignored mask (the flag cannot be removed by fanotify uapi).

Performance results for doing 10000000 write(2)s to tmpfs:

vanilla patched
without notification mark 25.486+-1.054 24.965+-0.244
with notification mark 30.111+-0.139 26.891+-1.355

So we can see the overhead of notification subsystem has been
drastically reduced.

Link: https://lore.kernel.org/r/[e-mail]
Suggested-by: Jan Kara <[e-mail]>
Signed-off-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/fanotify/fanotify_user.c

···
 				    flags, umask);
 }
 
+static void fanotify_mark_add_ignored_mask(struct fsnotify_mark *fsn_mark,
+					   __u32 mask, unsigned int flags,
+					   __u32 *removed)
+{
+	fsn_mark->ignored_mask |= mask;
+
+	/*
+	 * Setting FAN_MARK_IGNORED_SURV_MODIFY for the first time may lead to
+	 * the removal of the FS_MODIFY bit in calculated mask if it was set
+	 * because of an ignored mask that is now going to survive FS_MODIFY.
+	 */
+	if ((flags & FAN_MARK_IGNORED_SURV_MODIFY) &&
+	    !(fsn_mark->flags & FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY)) {
+		fsn_mark->flags |= FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY;
+		if (!(fsn_mark->mask & FS_MODIFY))
+			*removed = FS_MODIFY;
+	}
+}
+
 static __u32 fanotify_mark_add_to_mask(struct fsnotify_mark *fsn_mark,
-				       __u32 mask, unsigned int flags)
+				       __u32 mask, unsigned int flags,
+				       __u32 *removed)
 {
 	__u32 oldmask, newmask;
 
···
 	if (!(flags & FAN_MARK_IGNORED_MASK)) {
 		fsn_mark->mask |= mask;
 	} else {
-		fsn_mark->ignored_mask |= mask;
-		if (flags & FAN_MARK_IGNORED_SURV_MODIFY)
-			fsn_mark->flags |= FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY;
+		fanotify_mark_add_ignored_mask(fsn_mark, mask, flags, removed);
 	}
 	newmask = fsnotify_calc_mask(fsn_mark);
 	spin_unlock(&fsn_mark->lock);
···
 			     __kernel_fsid_t *fsid)
 {
 	struct fsnotify_mark *fsn_mark;
-	__u32 added;
+	__u32 added, removed = 0;
 	int ret = 0;
 
 	mutex_lock(&group->mark_mutex);
···
 			goto out;
 	}
 
-	added = fanotify_mark_add_to_mask(fsn_mark, mask, flags);
-	if (added & ~fsnotify_conn_mask(fsn_mark->connector))
+	added = fanotify_mark_add_to_mask(fsn_mark, mask, flags, &removed);
+	if (removed || (added & ~fsnotify_conn_mask(fsn_mark->connector)))
 		fsnotify_recalc_mask(fsn_mark->connector);
 
 out:

fs/notify/fsnotify.c

···
 
 
 	/*
-	 * if this is a modify event we may need to clear the ignored masks
-	 * otherwise return if none of the marks care about this type of event.
+	 * If this is a modify event we may need to clear some ignored masks.
+	 * In that case, the object with ignored masks will have the FS_MODIFY
+	 * event in its mask.
+	 * Otherwise, return if none of the marks care about this type of event.
 	 */
 	test_mask = (mask & ALL_FSNOTIFY_EVENTS);
-	if (!(mask & FS_MODIFY) && !(test_mask & marks_mask))
+	if (!(test_mask & marks_mask))
 		return 0;
 
 	iter_info.srcu_idx = srcu_read_lock(&fsnotify_mark_srcu);

include/linux/fsnotify_backend.h

···
 	if (!mark->ignored_mask)
 		return mask;
 
+	/* Interest in FS_MODIFY may be needed for clearing ignored mask */
+	if (!(mark->flags & FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY))
+		mask |= FS_MODIFY;
+
 	/*
 	 * If mark is interested in ignoring events on children, the object must
 	 * show interest in those events for fsnotify_parent() to notice it.