The patch titled
Subject: memcg: page_cgroup_ino() get memcg from compound_head(page)
has been added to the -mm mm-unstable branch. Its filename is
memcg-page_cgroup_ino-get-memcg-from-compound_headpage.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/memcg-page_cgroup_ino-get-memcg-from-compound_headpage.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: memcg: page_cgroup_ino() get memcg from compound_head(page)
Date: Mon, 13 Mar 2023 08:34:52 +0000
In a kernel with added WARN_ON_ONCE(PageTail) in page_memcg_check(), we
observed a warning from page_cgroup_ino() when reading /proc/kpagecgroup.
This warning was added to catch fragile reads of a page memcg. Make
page_cgroup_ino() get memcg from compound_head(page): that gives it the
correct memcg for each subpage of a compound page, so is the right fix.
I dithered between the right fix and the safer "fix": it's unlikely but
conceivable that some userspace has learnt that /proc/kpagecgroup gives no
memcg on tail pages, and compensates for that in some (racy) way: so
continuing to give no memcg on tails, without warning, might be safer.
But hwpoison_filter_task(), the only other user of page_cgroup_ino(),
persuaded me. It looks as if it currently leaves out tail pages of the
selected memcg, by mistake: whereas hwpoison_inject() uses compound_head()
and expects the tails to be included. So hwpoison testing coverage has
probably been restricted by the wrong output from page_cgroup_ino() (if
that memcg filter is used at all): in the short term, it might be safer
not to enable wider coverage there, but long term we would regret that.
Link: https://lkml.kernel.org/r/20230313083452.1319968-1-yosryahmed@xxxxxxxxxx
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/mm/memcontrol.c~memcg-page_cgroup_ino-get-memcg-from-compound_headpage
+++ a/mm/memcontrol.c
@@ -395,7 +395,7 @@ ino_t page_cgroup_ino(struct page *page)
unsigned long ino = 0;
rcu_read_lock();
- memcg = page_memcg_check(page);
+ memcg = page_memcg_check(compound_head(page));
while (memcg && !(memcg->css.flags & CSS_ONLINE))
memcg = parent_mem_cgroup(memcg);
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
memcg-page_cgroup_ino-get-memcg-from-compound_headpage.patch
