Re: [PATCH v3] mm: mglru: fix stale batch updates after memcg reparenting
From: Qi Zheng
Date: Fri Jun 26 2026 - 03:10:46 EST
On 6/26/26 2:48 PM, Harry Yoo wrote:
On 6/26/26 3:24 PM, Qi Zheng wrote:
On 6/26/26 12:59 PM, Harry Yoo wrote:
On 6/26/26 1:48 PM, Qi Zheng wrote:
On 6/26/26 12:43 PM, Harry Yoo wrote:
On 6/26/26 11:27 AM, Qi Zheng wrote:
Hi Johannes,
On 6/26/26 2:41 AM, Johannes Weiner wrote:
On Thu, Jun 25, 2026 at 11:15:54PM +0800, Qi Zheng wrote:
From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
The mglru page table walker batches per-generation size deltas in
walk->nr_pages while walking page tables without holding the lruvec
lock.
The reset_batch_size() later folds those deltas into walk->lruvec
under
the lruvec lock.
The page table walker can run concurrently with the memcg
reparenting
path
as follows:
CPU0 CPU1
==== ====
walk_mm
--> walk_page_range
--> update_batch_size
--> walk->nr_pages += delta
mem_cgroup_css_offline
--> memcg_reparent_objcgs
--> lock lruvec
lru_gen_reparent_memcg
--> reparent child
folios to
parent
unlock lruvec
lock lruvec
reset_batch_size
--> child lrugen->nr_pages += delta
This will trigger the following warning in lru_gen_exit_memcg():
VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
sizeof(lruvec->lrugen.nr_pages)));
And the user-visible impact of underestimated nr_pages in MGLRU was
premature OOMs because MGLRU does not try to reclaim memory when
nr_pages
reaches zero, but there are still more pages.
To fix it, make reset_batch_size() check CSS_DYING under RCU before
flushing the pending batch. A non-dying memcg keeps the original
lruvec
stable against RCU-delayed offlining; a dying memcg redirects the
deltas
to the first non-dying ancestor.
Reported-by: Peiyang He <peiyang_he@xxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-
efb8-4b9a-a53a-1e28cc894f0b@xxxxxxxxxxxxxxxx
Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU
folios")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
Changes in v3:
- re-implement lock_batch_lruvec() by checking CSS_DYING
under the
RCU lock
(suggested by Harry)
- update the commit message (suggested by Harry)
- temporarily drop the previous Reviewed-by tags
(since the sync method has changed)
- rebase onto the next-20260624
Changes in v2:
- update the commit message (pointed by Barry)
- collect Reviewed-by
mm/vmscan.c | 45 ++++++++++++++++++++++++++++++++++++++-------
1 file changed, 38 insertions(+), 7 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 35c3bb15ae96..1ec8c23c72b9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3262,10 +3262,44 @@ static void update_batch_size(struct
lru_gen_mm_walk *walk, struct folio *folio,
walk->nr_pages[new_gen][type][zone] += delta;
}
+#ifdef CONFIG_MEMCG
+static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec)
+{
+ struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+
+ rcu_read_lock();
Where is this unlocked?
The lruvec_unlock_irq() in reset_batch_size() will handle the
unlocking.
+ /*
+ * The memcg can be NULL when the memory controller is
disabled.
+ * Otherwise, the caller keeps the memcg owning @lruvec alive.
+ */
+ if (!memcg || !css_is_dying(&memcg->css))
+ goto lock;
+
+ do {
+ memcg = parent_mem_cgroup(memcg);
+ } while (memcg && css_is_dying(&memcg->css));
+ lruvec = mem_cgroup_lruvec(memcg, pgdat);
while (unlikely(memcg && css_is_dying(&memcg->css))) {
memcg = parent_mem_cgroup(memcg);
lruvec = mem_cgroup_lruvec(memcg, pgdat);
There is no need to acquire the lruvec before finding the first
non-dying memcg.
struct pglist_data *pgdat = lruvec_pgdat(lruvec);
struct mem_cgroup *memcg = lruvec_memcg(lruvec);
rcu_read_lock()
while (unlikely(memcg_is_dying(memcg)))
memcg = parent_mem_cgroup(memcg);
lruvec = mem_cgroup_lruvec(memcg, pgdat);
If the first memcg is already non-dying, there's no need to re-acquire
the lruvec. ;)
Oh, right :)
Hmm but I still think Johannes' suggestion makes the code cleaner.
I don't have a strong preference on which of the two coding styles is
more readable. BTW, is there any kernel documentation I could refer to
for this?
I don't think there's a coding style guide that specifically
mentions this. Just thought it's cleaner because it merges if (...) goto
lock; and do-while into a single while loop.
Observing a dying cgroup should be rare anyway, it's worth focusing
more on readability?
While it's rare to encounter consecutive dying memcgs, it can still
happen, right?
But is worth saving a few instruction in a basic block that is
unlikely() to be executed?
I don't have a strong opinion here. Hi Johannes, I'll leave the decision
up to you. If necessary, I can send out the v4.
I'm not a memcg maintainer myself, though. just my 2 cents.
I'd like to express my gratitude for your reviews, and especially
for your invaluable help with the earlier dying memcg work!
Thanks,
Qi