[Devel] [PATCH RHEL9 COMMIT] memcg: Add a comment on using page_memcg() in cleancache code

[Konstantin Khorenko]

The commit is pushed to "branch-rh9-5.14.vz9.1.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh9-5.14.0-4.vz9.10.13
------>
commit ef37d176e210c32a23e65236d3086e34a7f762ed
Author: Nikita Yushchenko <nikita.yushchenko at virtuozzo.com>
Date:   Wed Oct 20 18:07:51 2021 +0300

    memcg: Add a comment on using page_memcg() in cleancache code
    
    To_merge: e94505ccca99 ("memcg: allow to disable cleancache per memcg")
    
    Signed-off-by: Nikita Yushchenko <nikita.yushchenko at virtuozzo.com>
---
 mm/memcontrol.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f63f998ceccb..19a5638ce1dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1322,6 +1322,16 @@ bool mem_cgroup_cleancache_disabled(struct page *page)
 	if (mem_cgroup_disabled())
 		return false;
 
+	/*
+	 * On all paths to mem_cgroup_cleancache_disabled(), the page in
+	 * question is locked. Calls go from __delete_from_page_cache() or
+	 * from delete_from_page_cache_batch(), in both cases page lock is
+	 * mandatory.
+	 *
+	 * Per comment in the documentation of page_memcg(), holding page lock
+	 * guarantees stability of the value returned from page_memcg().
+	 * Thus using page_memcg() is safe, rcu is not needed.
+	 */
 	memcg = page_memcg(page);
 	return memcg && memcg->cleancache_disabled;
 }