[PATCH] mm/huge_memory: fix swap entry values of tail pages of THP

[Charan Teja Kalla <quic_charante@quicinc.com>]

An anon THP page is first added to swap cache before reclaiming it.
Initially, each tail page contains the proper swap entry value(stored in
->private field) which is filled from add_to_swap_cache(). After
migrating the THP page sitting on the swap cache, only the swap entry of
the head page is filled(see folio_migrate_mapping()).

Now when this page is tried to split(one case is when this page is again
migrated, see migrate_pages()->try_split_thp()), the tail pages
->private is not stored with proper swap entry values.  When this tail
page is now try to be freed, as part of it delete_from_swap_cache() is
called which operates on the wrong swap cache index and eventually
replaces the wrong swap cache index with shadow/NULL value, frees the
page.

This leads to the state with a swap cache containing the freed page.
This issue can manifest in many forms and the most common thing observed
is the rcu stall during the swapin (see mapping_get_entry()).

On the recent kernels, this issues is indirectly getting fixed with the
series[1], to be specific[2].

When tried to back port this series, it is observed many merge
conflicts and also seems dependent on many other changes. As backporting
to LTS branches is not a trivial one, the similar change from [2] is
picked as a fix.

[1] https://lore.kernel.org/all/20230821160849.531668-1-david@redhat.com/
[2] https://lore.kernel.org/all/20230821160849.531668-5-david@redhat.com/

Closes: https://lore.kernel.org/linux-mm/69cb784f-578d-ded1-cd9f-c6db04696336@quicinc.com/
Fixes: 3417013e0d18 ("mm/migrate: Add folio_migrate_mapping()")
Cc: <stable@vger.kernel.org> # see patch description, applicable to <=6.1
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
---
 mm/huge_memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 5957794..cc5273f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2477,6 +2477,8 @@ static void __split_huge_page_tail(struct page *head, int tail,
 	if (!folio_test_swapcache(page_folio(head))) {
 		VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
 		page_tail->private = 0;
+	} else {
+		set_page_private(page_tail, (unsigned long)head->private + tail);
 	}
 
 	/* Page flags must be visible before we make the page non-compound. */
-- 
2.7.4