From: Vlastimil Babka <vbabka@xxxxxxx> Subject: mm, slab, slub: clear the slab_cache field when freeing page The page allocator expects that page->mapping is NULL for a page being freed. SLAB and SLUB use the slab_cache field which is in union with mapping, but before freeing the page, the field is referenced with the "mapping" name when set to NULL. It's IMHO more correct (albeit functionally the same) to use the slab_cache name as that's the field we use in SL*B, and document why we clear it in a comment (we don't clear fields such as s_mem or freelist, as page allocator doesn't care about those). While using the 'mapping' name would automagically keep the code correct if the unions in struct page changed, such changes should be done consciously and needed changes evaluated - the comment should help with that. Link: https://lkml.kernel.org/r/20201210160020.21562-1-vbabka@xxxxxxx Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Acked-by: David Rientjes <rientjes@xxxxxxxxxx> Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/slab.c | 3 ++- mm/slub.c | 4 ++-- 2 files changed, 4 insertions(+), 3 deletions(-) --- a/mm/slab.c~mm-slab-slub-clear-the-slab_cache-field-when-freeing-page +++ a/mm/slab.c @@ -1399,7 +1399,8 @@ static void kmem_freepages(struct kmem_c __ClearPageSlabPfmemalloc(page); __ClearPageSlab(page); page_mapcount_reset(page); - page->mapping = NULL; + /* In union with page->mapping where page allocator expects NULL */ + page->slab_cache = NULL; if (current->reclaim_state) current->reclaim_state->reclaimed_slab += 1 << order; --- a/mm/slub.c~mm-slab-slub-clear-the-slab_cache-field-when-freeing-page +++ a/mm/slub.c @@ -1836,8 +1836,8 @@ static void __free_slab(struct kmem_cach __ClearPageSlabPfmemalloc(page); __ClearPageSlab(page); - - page->mapping = NULL; + /* In union with page->mapping where page allocator expects NULL */ + page->slab_cache = NULL; if (current->reclaim_state) current->reclaim_state->reclaimed_slab += pages; unaccount_slab_page(page, order, s); _
