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@xxxxxxxxxx/ [2] https://lore.kernel.org/all/20230821160849.531668-5-david@xxxxxxxxxx/ Closes: https://lore.kernel.org/linux-mm/69cb784f-578d-ded1-cd9f-c6db04696336@xxxxxxxxxxx/ Fixes: 3417013e0d18 ("mm/migrate: Add folio_migrate_mapping()") Cc: <stable@xxxxxxxxxxxxxxx> # see patch description, applicable to <=6.1 Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx> --- 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
