[Devel] [PATCH 3/4] don't do __ClearPageSlab before freeing slab page.

[Glauber Costa]

This will give the oportunity to the page allocator to
determine that a given page was previously a slab page, and
take action accordingly.

If memcg kmem is present, this means that that page needs to
be unaccounted. The page allocator will now have the responsibility
to clear that bit upon free_pages().

It is not uncommon to have the page allocator to check page flags.
Mlock flag, for instance, is checked pervasively all over the place.
So I hope this is okay for the slab as well.

Signed-off-by: Glauber Costa <glommer at parallels.com>
CC: Pekka Enberg <penberg at cs.helsinki.fi>
CC: Michal Hocko <mhocko at suse.cz>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
CC: Johannes Weiner <hannes at cmpxchg.org>
CC: Suleiman Souhlal <suleiman at google.com>
---
 mm/page_alloc.c |    5 ++++-
 mm/slab.c       |    5 -----
 mm/slob.c       |    1 -
 mm/slub.c       |    1 -
 4 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 918330f..a884a9c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -697,8 +697,10 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
 
 	if (PageAnon(page))
 		page->mapping = NULL;
-	for (i = 0; i < (1 << order); i++)
+	for (i = 0; i < (1 << order); i++) {
+		__ClearPageSlab(page + i);
 		bad += free_pages_check(page + i);
+	}
 	if (bad)
 		return false;
 
@@ -2505,6 +2507,7 @@ EXPORT_SYMBOL(get_zeroed_page);
 void __free_pages(struct page *page, unsigned int order)
 {
 	if (put_page_testzero(page)) {
+		__ClearPageSlab(page);
 		if (order == 0)
 			free_hot_cold_page(page, 0);
 		else
diff --git a/mm/slab.c b/mm/slab.c
index d7dfd26..66ef370 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1794,11 +1794,6 @@ static void kmem_freepages(struct kmem_cache *cachep, void *addr)
 	else
 		sub_zone_page_state(page_zone(page),
 				NR_SLAB_UNRECLAIMABLE, nr_freed);
-	while (i--) {
-		BUG_ON(!PageSlab(page));
-		__ClearPageSlab(page);
-		page++;
-	}
 	if (current->reclaim_state)
 		current->reclaim_state->reclaimed_slab += nr_freed;
 	free_pages((unsigned long)addr, cachep->gfporder);
diff --git a/mm/slob.c b/mm/slob.c
index 61b1845..b03d65e 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -360,7 +360,6 @@ static void slob_free(void *block, int size)
 		if (slob_page_free(sp))
 			clear_slob_page_free(sp);
 		spin_unlock_irqrestore(&slob_lock, flags);
-		__ClearPageSlab(sp);
 		reset_page_mapcount(sp);
 		slob_free_pages(b, 0);
 		return;
diff --git a/mm/slub.c b/mm/slub.c
index ed01be5..a0eeb4a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1399,7 +1399,6 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
 		-pages);
 
-	__ClearPageSlab(page);
 	reset_page_mapcount(page);
 	if (current->reclaim_state)
 		current->reclaim_state->reclaimed_slab += pages;
-- 
1.7.10.2