[PATCH] memcg: page_cgroup_ino() get memcg from compound_head(page)

[Yosry Ahmed <yosryahmed@google.com>]

From: Hugh Dickins <hughd@google.com>

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.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
---

(Yosry: Alternatively, we could modify page_memcg_check() to do
 page_folio() like its sibling page_memcg(), as page_cgroup_ino() is the
 only remaining caller other than print_page_owner_memcg(); and it already
 excludes pages that have page->memcg_data = 0)

---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5abffe6f8389..e3a55295725e 100644
--- a/mm/memcontrol.c
+++ b/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);
-- 
2.40.0.rc1.284.g88254d51c5-goog