On Tue, Feb 13, 2024 at 02:18:10PM +0530, Charan Teja Kalla wrote: > 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/ I am deeply confused by this commit message. Are you saying there is a problem in current HEAD which this fixes, or are you saying that this problem has already been fixed, and this patch is for older kernels? > 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 >
