fanotify: limit number of event merge attempts

stable inclusion
from stable-5.10.67
commit ad3ea16746cc68307165d52e22a3f75015ccd16e
bugzilla: 182619 https://gitee.com/openeuler/kernel/issues/I4EWO7

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad3ea16746cc68307165d52e22a3f75015ccd16e

--------------------------------

commit b8cd0ee8 upstream.

Event merges are expensive when event queue size is large, so limit the
linear search to 128 merge tests.

[Stable backport notes] The following statement from upstream commit is
irrelevant for backport:
-
-In combination with 128 size hash table, there is a potential to merge
-with up to 16K events in the hashed queue.
-
[Stable backport notes] The problem is as old as fanotify and described
in the linked cover letter "Performance improvement for fanotify merge".
This backported patch fixes the performance issue at the cost of merging
fewer potential events.  Fixing the performance issue is more important
than preserving the "event merge" behavior, which was not predictable in
any way that applications could rely on.

Link: https://lore.kernel.org/r/20210304104826.3993892-6-amir73il@gmail.com


Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/linux-fsdevel/20210202162010.305971-1-amir73il@gmail.com/
Link: https://lore.kernel.org/linux-fsdevel/20210915163334.GD6166@quack2.suse.cz/


Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Weilong Chen <chenweilong@huawei.com>

Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>