Re: [PATCH mm-unstable v3 4/5] mm: restart if multiple traversals raced

[Hugh Dickins <hughd@google.com>]

On Tue, 27 Aug 2024, Kinsey Ho wrote:

> Currently, if multiple reclaimers raced on the same position, the
> reclaimers which detect the race will still reclaim from the same memcg.
> Instead, the reclaimers which detect the race should move on to the next
> memcg in the hierarchy.
> 
> So, in the case where multiple traversals race, jump back to the start
> of the mem_cgroup_iter() function to find the next memcg in the
> hierarchy to reclaim from.
> 
> Signed-off-by: Kinsey Ho <kinseyho@google.com>

mm-unstable commit 954dd0848c61 needs the fix below to be merged in;
but the commit after it (the 5/5) then renames "memcg" to "next",
so that one has to be adjusted too.

[PATCH] mm: restart if multiple traversals raced: fix

mem_cgroup_iter() reset memcg to NULL before the goto restart, so that
goto out_unlock does not then return an ungotten memcg, causing oopses
on stale memcg in many places (often in memcg_rstat_updated()).

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/memcontrol.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6f66ac0ad4f0..dd82dd1e1f0a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1049,6 +1049,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		if (cmpxchg(&iter->position, pos, memcg) != pos) {
 			if (css && css != &root->css)
 				css_put(css);
+			memcg = NULL;
 			goto restart;
 		}
 
-- 
2.35.3