[PATCH] memcg: page_cgroup_ino() get memcg from the page's folio

[Yosry Ahmed <yosryahmed@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 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.

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

This is based on a patch originally written by Hugh Dickins and retains
most of the original commit log:
https://lore.kernel.org/linux-mm/20230313083452.1319968-1-yosryahmed@google.com/

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.

I dropped Michal's Ack as the only line in the patch was changed, but
the patch should be functionally the same.

---
 mm/memcontrol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5abffe6f8389..fec3c4fd9c1c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -395,7 +395,8 @@ 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);
-- 
2.40.0.577.gac1e443424-goog