The patch titled
Subject: memcg: page_cgroup_ino() get memcg from the page's folio
has been added to the -mm mm-unstable branch. Its filename is
memcg-page_cgroup_ino-get-memcg-from-the-pages-folio.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-the-pages-folio.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: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Subject: memcg: page_cgroup_ino() get memcg from the page's folio
Date: Wed, 12 Apr 2023 00:34:51 +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 the page's folio using
folio_memcg_check(): that gives it the correct memcg for each page of a
folio, so is the right fix.
Note that page_folio() is racy, the page's folio can change from under us,
but the entire function is racy and documented as such.
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.
This is based on a patch originally written by Hugh Dickins and retains
most of the original commit log [1]
The patch was changed to use folio_memcg_check(page_folio(page)) instead
of page_memcg_check(compound_head(page)) based on discussions with Matthew
Wilcox; where he stated that callers of page_memcg_check() should stop
using it due to the ambiguity around tail pages -- instead they should use
folio_memcg_check() and handle tail pages themselves.
Link: https://lkml.kernel.org/r/20230412003451.4018887-1-yosryahmed@xxxxxxxxxx
Link: https://lore.kernel.org/linux-mm/20230313083452.1319968-1-yosryahmed@xxxxxxxxxx/ [1]
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
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>
---
mm/memcontrol.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/mm/memcontrol.c~memcg-page_cgroup_ino-get-memcg-from-the-pages-folio
+++ a/mm/memcontrol.c
@@ -396,7 +396,8 @@ ino_t page_cgroup_ino(struct page *page)
unsigned long ino = 0;
rcu_read_lock();
- memcg = page_memcg_check(page);
+ /* page_folio() is racy here, but the entire function is racy anyway */
+ memcg = folio_memcg_check(page_folio(page));
while (memcg && !(memcg->css.flags & CSS_ONLINE))
memcg = parent_mem_cgroup(memcg);
_
Patches currently in -mm which might be from yosryahmed@xxxxxxxxxx are
cgroup-rename-cgroup_rstat_flush_irqsafe-to-atomic.patch
memcg-rename-mem_cgroup_flush_stats_delayed-to-ratelimited.patch
memcg-do-not-flush-stats-in-irq-context.patch
memcg-replace-stats_flush_lock-with-an-atomic.patch
memcg-sleep-during-flushing-stats-in-safe-contexts.patch
workingset-memcg-sleep-when-flushing-stats-in-workingset_refault.patch
vmscan-memcg-sleep-when-flushing-stats-during-reclaim.patch
memcg-do-not-modify-rstat-tree-for-zero-updates.patch
mm-vmscan-move-set_task_reclaim_state-after-global_reclaim.patch
mm-vmscan-refactor-updating-reclaimed-pages-in-reclaim_state.patch
mm-vmscan-ignore-non-lru-based-reclaim-in-memcg-reclaim.patch
memcg-page_cgroup_ino-get-memcg-from-the-pages-folio.patch
