+ mm-slab-slub-clear-the-slab_cache-field-when-freeing-page.patch added to -mm tree

+ mm-slab-slub-clear-the-slab_cache-field-when-freeing-page.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm, slab, slub: clear the slab_cache field when freeing page
has been added to the -mm tree.  Its filename is
     mm-slab-slub-clear-the-slab_cache-field-when-freeing-page.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-slab-slub-clear-the-slab_cache-field-when-freeing-page.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-slab-slub-clear-the-slab_cache-field-when-freeing-page.patch

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------------------------------------------------------
From: Vlastimil Babka <vbabka@suse.cz>
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@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 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);
_

Patches currently in -mm which might be from vbabka@suse.cz are

mm-slab-slub-clear-the-slab_cache-field-when-freeing-page.patch
mm-slub-use-kmem_cache_debug_flags-in-deactivate_slab.patch
mm-page_alloc-clean-up-pageset-high-and-batch-update.patch
mm-page_alloc-calculate-pageset-high-and-batch-once-per-zone.patch
mm-page_alloc-remove-setup_pageset.patch
mm-page_alloc-simplify-pageset_update.patch
mm-page_alloc-cache-pageset-high-and-batch-in-struct-zone.patch
mm-page_alloc-move-draining-pcplists-to-page-isolation-users.patch
mm-page_alloc-disable-pcplists-during-memory-offline.patch
mm-page_alloc-disable-pcplists-during-memory-offline-fix.patch
mm-page_alloc-do-not-rely-on-the-order-of-page_poison-and-init_on_alloc-free-parameters.patch
mm-page_poison-use-static-key-more-efficiently.patch
kernel-power-allow-hibernation-with-page_poison-sanity-checking.patch
mm-page_poison-remove-config_page_poisoning_no_sanity.patch
mm-page_poison-remove-config_page_poisoning_zero.patch