6.12-stable review patch. If anyone has any objections, please let me know.
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From: Wei Yang richard.weiyang@gmail.com
[ Upstream commit cff47b9e39a6abf03dde5f4f156f841b0c54bba0 ]
Commit c010d47f107f ("mm: thp: split huge page to any lower order pages") introduced an early check on the folio's order via mapping->flags before proceeding with the split work.
This check introduced a bug: for shmem folios in the swap cache and truncated folios, the mapping pointer can be NULL. Accessing mapping->flags in this state leads directly to a NULL pointer dereference.
This commit fixes the issue by moving the check for mapping != NULL before any attempt to access mapping->flags.
Link: https://lkml.kernel.org/r/20251119235302.24773-1-richard.weiyang@gmail.com Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages") Signed-off-by: Wei Yang richard.weiyang@gmail.com Reviewed-by: Zi Yan ziy@nvidia.com Acked-by: David Hildenbrand (Red Hat) david@kernel.org Reviewed-by: Baolin Wang baolin.wang@linux.alibaba.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org [ applied fix to split_huge_page_to_list_to_order() instead of __folio_split() ] Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/huge_memory.c | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-)
--- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3404,6 +3404,16 @@ int split_huge_page_to_list_to_order(str if (new_order >= folio_order(folio)) return -EINVAL;
+ /* + * Folios that just got truncated cannot get split. Signal to the + * caller that there was a race. + * + * TODO: this will also currently refuse shmem folios that are in the + * swapcache. + */ + if (!is_anon && !folio->mapping) + return -EBUSY; + if (is_anon) { /* order-1 is not supported for anonymous THP. */ if (new_order == 1) { @@ -3466,13 +3476,6 @@ int split_huge_page_to_list_to_order(str gfp_t gfp;
mapping = folio->mapping; - - /* Truncated ? */ - if (!mapping) { - ret = -EBUSY; - goto out; - } - min_order = mapping_min_folio_order(folio->mapping); if (new_order < min_order) { VM_WARN_ONCE(1, "Cannot split mapped folio below min-order: %u",