mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

When I did a large folios split test, a WARNING "[ 5059.122759][ T166]
Cannot split file folio to non-0 order" was triggered.  But the test cases
are only for anonmous folios.  while mapping_large_folio_support() is only
reasonable for page cache folios.

In split_huge_page_to_list_to_order(), the folio passed to
mapping_large_folio_support() maybe anonmous folio.  The folio_test_anon()
check is missing.  So the split of the anonmous THP is failed.  This is
also the same for shmem_mapping().  We'd better add a check for both.  But
the shmem_mapping() in __split_huge_page() is not involved, as for
anonmous folios, the end parameter is set to -1, so (head[i].index >= end)
is always false.  shmem_mapping() is not called.

Also add a VM_WARN_ON_ONCE() in mapping_large_folio_support() for anon
mapping, So we can detect the wrong use more easily.

THP folios maybe exist in the pagecache even the file system doesn't
support large folio, it is because when CONFIG_TRANSPARENT_HUGEPAGE is
enabled, khugepaged will try to collapse read-only file-backed pages to
THP.  But the mapping does not actually support multi order large folios
properly.

Using /sys/kernel/debug/split_huge_pages to verify this, with this patch,
large anon THP is successfully split and the warning is ceased.

Link: https://lkml.kernel.org/r/202406071740485174hcFl7jRxncsHDtI-Pz-o@zte.com.cn
Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index ee633712bba0..59f1df0cde5a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -381,6 +381,10 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
  */
 static inline bool mapping_large_folio_support(struct address_space *mapping)
 {
+	/* AS_LARGE_FOLIO_SUPPORT is only reasonable for pagecache folios */
+	VM_WARN_ONCE((unsigned long)mapping & PAGE_MAPPING_ANON,
+			"Anonymous mapping always supports large folio");
+
 	return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
 		test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
 }