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

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/[email protected]
Link: https://lore.kernel.org/linux-mm/[email protected]/ [1]
Signed-off-by: Yosry Ahmed <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions