On 2025/3/6 4:04, Zi Yan wrote: > A shmem folio can be either in page cache or in swap cache, but not at the > same time. Namely, once it is in swap cache, folio->mapping should be NULL, > and the folio is no longer in a shmem mapping. > > In __folio_migrate_mapping(), to determine the number of xarray entries > to update, folio_test_swapbacked() is used, but that conflates shmem in > page cache case and shmem in swap cache case. It leads to xarray > multi-index entry corruption, since it turns a sibling entry to a > normal entry during xas_store() (see [1] for a userspace reproduction). > Fix it by only using folio_test_swapcache() to determine whether xarray > is storing swap cache entries or not to choose the right number of xarray > entries to update. > > [1] https://lore.kernel.org/linux-mm/Z8idPCkaJW1IChjT@xxxxxxxxxxxxxxxxxxxx/ > > Note: > In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is used > to get swap_cache address space, but that ignores the shmem folio in swap > cache case. It could lead to NULL pointer dereferencing when a > in-swap-cache shmem folio is split at __xa_store(), since > !folio_test_anon() is true and folio->mapping is NULL. But fortunately, > its caller split_huge_page_to_list_to_order() bails out early with EBUSY > when folio->mapping is NULL. So no need to take care of it here. > > Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly") > Reported-by: Liu Shixin <liushixin2@xxxxxxxxxx> > Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@xxxxxxxxxx/ > Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx> > Signed-off-by: Zi Yan <ziy@xxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx Thanks for the patch, it works for me. > --- > mm/migrate.c | 10 ++++------ > 1 file changed, 4 insertions(+), 6 deletions(-) > > diff --git a/mm/migrate.c b/mm/migrate.c > index fb4afd31baf0..c0adea67cd62 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -518,15 +518,13 @@ static int __folio_migrate_mapping(struct address_space *mapping, > if (folio_test_anon(folio) && folio_test_large(folio)) > mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1); > folio_ref_add(newfolio, nr); /* add cache reference */ > - if (folio_test_swapbacked(folio)) { > + if (folio_test_swapbacked(folio)) > __folio_set_swapbacked(newfolio); > - if (folio_test_swapcache(folio)) { > - folio_set_swapcache(newfolio); > - newfolio->private = folio_get_private(folio); > - } > + if (folio_test_swapcache(folio)) { > + folio_set_swapcache(newfolio); > + newfolio->private = folio_get_private(folio); > entries = nr; > } else { > - VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio); > entries = 1; > } >
