On 6/19/20 4:33 PM, Greg Kroah-Hartman wrote:

From: Andrea Arcangeli <redacted>

commit c444eb564fb16645c172d550359cb3d75fe8a040 upstream.

Write protect anon page faults require an accurate mapcount to decide
if to break the COW or not. This is implemented in the THP path with
reuse_swap_page() ->
page_trans_huge_map_swapcount()/page_trans_huge_mapcount().

If the COW triggers while the other processes sharing the page are
under a huge pmd split, to do an accurate reading, we must ensure the
mapcount isn't computed while it's being transferred from the head
page to the tail pages.

reuse_swap_cache() already runs serialized by the page lock, so it's
enough to add the page lock around __split_huge_pmd_locked too, in
order to add the missing serialization.

Note: the commit in "Fixes" is just to facilitate the backporting,
because the code before such commit didn't try to do an accurate THP
mapcount calculation and it instead used the page_count() to decide if
to COW or not. Both the page_count and the pin_count are THP-wide
refcounts, so they're inaccurate if used in
reuse_swap_page(). Reverting such commit (besides the unrelated fix to
the local anon_vma assignment) would have also opened the window for
memory corruption side effects to certain workloads as documented in
such commit header.

Signed-off-by: Andrea Arcangeli <redacted>
Suggested-by: Jann Horn <jannh@google.com>
Reported-by: Jann Horn <jannh@google.com>
Acked-by: Kirill A. Shutemov <redacted>
Fixes: 6d0a07edd17c ("mm: thp: calculate the mapcount correctly for THP pages during WP faults")
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Hi, when evaluating this backport for our 4.12 based kernel, Nicolai found out
that Jann's POC still triggers, AFAICS because do_huge_pmd_wp_page() doesn't
take the page lock, which was only added by ba3c4ce6def4 ("mm, THP, swap: make
reuse_swap_page() works for THP swapped out") in 4.14. The upstream stable 4.9
is thus in the same situation (didn't actually test the POC there, but should be
obvious), so this is a heads up.

Now just backporting ba3c4ce6def4 to 4.9 stable isn't that simple, as that's
part of a larger series (maybe with even more prerequisities, didn't check). I'm
considering just taking the part of ba3c4ce6def4 that's wrapping
page_trans_huge_mapcount() in the page lock (without changing it to
reuse_swap_page() and changing the latter to deal with swapped out THP) and will
look at it tomorrow. But suggestions (and/or later review) from Andrea/Kirill
are welcome.

Thanks,
Vlastimil