Thread (9 messages) flat view 9 messages, 2 authors, 2012-09-07

Re: [PATCH 1/4] slab: do ClearSlabPfmemalloc() for all pages of slab

From: Mel Gorman <mgorman@suse.de>
Date: 2012-09-07 12:55:27
Also in: linux-mm, lkml
Subsystem: memory management, the rest · Maintainers: Andrew Morton, Linus Torvalds 

On Fri, Sep 07, 2012 at 03:05:39AM +0900, JoonSoo Kim wrote:
Correct Pekka's mail address and resend.
Sorry.

Add "Cc" to "Christoph Lameter" [off-list ref]

2012/9/5 Mel Gorman [off-list ref]:
quoted
Right now, we call ClearSlabPfmemalloc() for first page of slab when we
clear SlabPfmemalloc flag. This is fine for most swap-over-network use
cases as it is expected that order-0 pages are in use. Unfortunately it
is possible that that __ac_put_obj() checks SlabPfmemalloc on a tail page
and while this is harmless, it is sloppy. This patch ensures that the head
page is always used.

This problem was originally identified by Joonsoo Kim.

[js1304@gmail.com: Original implementation and problem identification]
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/slab.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 811af03..d34a903 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1000,7 +1000,7 @@ static void *__ac_get_obj(struct kmem_cache *cachep, struct array_cache *ac,
                l3 = cachep->nodelists[numa_mem_id()];
                if (!list_empty(&l3->slabs_free) && force_refill) {
                        struct slab *slabp = virt_to_slab(objp);
-                       ClearPageSlabPfmemalloc(virt_to_page(slabp->s_mem));
+                       ClearPageSlabPfmemalloc(virt_to_head_page(slabp->s_mem));
                        clear_obj_pfmemalloc(&objp);
                        recheck_pfmemalloc_active(cachep, ac);
                        return objp;
We assume that slabp->s_mem's address is always in head page, so
"virt_to_head_page" is not needed.
Fair point. I thought it would be more "obvious" later that we really
always intended to use the head page but it is unnecessary.
quoted hunk ↗ jump to hunk
quoted
@@ -1032,7 +1032,7 @@ static void *__ac_put_obj(struct kmem_cache *cachep, struct array_cache *ac,
 {
        if (unlikely(pfmemalloc_active)) {
                /* Some pfmemalloc slabs exist, check if this is one */
-               struct page *page = virt_to_page(objp);
+               struct page *page = virt_to_head_page(objp);
                if (PageSlabPfmemalloc(page))
                        set_obj_pfmemalloc(&objp);
        }
--
1.7.9.2
If we always use head page, following suggestion is more good to me.
How about you?

diff --git a/mm/slab.c b/mm/slab.c
index f8b0d53..ce70989 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1032,7 +1032,7 @@ static void *__ac_put_obj(struct kmem_cache
*cachep, struct array_cache *ac,
 {
        if (unlikely(pfmemalloc_active)) {
                /* Some pfmemalloc slabs exist, check if this is one */
-               struct page *page = virt_to_page(objp);
+               struct page *page = virt_to_head_page(objp);
                if (PageSlabPfmemalloc(page))
                        set_obj_pfmemalloc(&objp);
        }
ok.
quoted hunk ↗ jump to hunk
@@ -1921,10 +1921,9 @@ static void *kmem_getpages(struct kmem_cache
*cachep, gfp_t flags, int nodeid)
                        NR_SLAB_UNRECLAIMABLE, nr_pages);
        for (i = 0; i < nr_pages; i++) {
                __SetPageSlab(page + i);
-
-               if (page->pfmemalloc)
-                       SetPageSlabPfmemalloc(page + i);
        }
+       if (page->pfmemalloc)
+               SetPageSlabPfmemalloc(page);

        if (kmemcheck_enabled && !(cachep->flags & SLAB_NOTRACK)) {
                kmemcheck_alloc_shadow(page, cachep->gfporder, flags, nodeid);
ok.
quoted hunk ↗ jump to hunk
@@ -1943,26 +1942,26 @@ static void *kmem_getpages(struct kmem_cache
*cachep, gfp_t flags, int nodeid)
  */
 static void kmem_freepages(struct kmem_cache *cachep, void *addr)
 {
-       unsigned long i = (1 << cachep->gfporder);
+       int nr_pages = (1 << cachep->gfporder);
+       int i;
        struct page *page = virt_to_page(addr);
-       const unsigned long nr_freed = i;

        kmemcheck_free_shadow(page, cachep->gfporder);

        if (cachep->flags & SLAB_RECLAIM_ACCOUNT)
                sub_zone_page_state(page_zone(page),
-                               NR_SLAB_RECLAIMABLE, nr_freed);
+                               NR_SLAB_RECLAIMABLE, nr_pages);
        else
                sub_zone_page_state(page_zone(page),
-                               NR_SLAB_UNRECLAIMABLE, nr_freed);
-       while (i--) {
-               BUG_ON(!PageSlab(page));
-               __ClearPageSlabPfmemalloc(page);
-               __ClearPageSlab(page);
-               page++;
+                               NR_SLAB_UNRECLAIMABLE, nr_pages);
+       for (i = 0; i < nr_pages; i++) {
+               BUG_ON(!PageSlab(page + i));
+               __ClearPageSlab(page + i);
        }
+       __ClearPageSlabPfmemalloc(page);
+
        if (current->reclaim_state)
-               current->reclaim_state->reclaimed_slab += nr_freed;
+               current->reclaim_state->reclaimed_slab += nr_pages;
        free_pages((unsigned long)addr, cachep->gfporder);
 }
This churns code a lot more than is necessary. How about this as a
replacement patch?

---8<---
From: Joonsoo Kim <redacted>
Subject: [PATCH] slab: do ClearSlabPfmemalloc() for all pages of slab

Right now, we call ClearSlabPfmemalloc() for first page of slab when we
clear SlabPfmemalloc flag. This is fine for most swap-over-network use
cases as it is expected that order-0 pages are in use. Unfortunately it
is possible that that __ac_put_obj() checks SlabPfmemalloc on a tail page
and while this is harmless, it is sloppy. This patch ensures that the head
page is always used.

[mgorman@suse.de: Easier implementation, changelog cleanup]
Signed-off-by: Joonsoo Kim <redacted>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/slab.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 811af03..590d52a 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1032,7 +1032,7 @@ static void *__ac_put_obj(struct kmem_cache *cachep, struct array_cache *ac,
 {
 	if (unlikely(pfmemalloc_active)) {
 		/* Some pfmemalloc slabs exist, check if this is one */
-		struct page *page = virt_to_page(objp);
+		struct page *page = virt_to_head_page(objp);
 		if (PageSlabPfmemalloc(page))
 			set_obj_pfmemalloc(&objp);
 	}
@@ -1919,12 +1919,10 @@ static void *kmem_getpages(struct kmem_cache *cachep, gfp_t flags, int nodeid)
 	else
 		add_zone_page_state(page_zone(page),
 			NR_SLAB_UNRECLAIMABLE, nr_pages);
-	for (i = 0; i < nr_pages; i++) {
+	for (i = 0; i < nr_pages; i++)
 		__SetPageSlab(page + i);
-
-		if (page->pfmemalloc)
-			SetPageSlabPfmemalloc(page + i);
-	}
+	if (page->pfmemalloc)
+		SetPageSlabPfmemalloc(page);
 
 	if (kmemcheck_enabled && !(cachep->flags & SLAB_NOTRACK)) {
 		kmemcheck_alloc_shadow(page, cachep->gfporder, flags, nodeid);
@@ -1955,9 +1953,9 @@ static void kmem_freepages(struct kmem_cache *cachep, void *addr)
 	else
 		sub_zone_page_state(page_zone(page),
 				NR_SLAB_UNRECLAIMABLE, nr_freed);
+	__ClearPageSlabPfmemalloc(page);
 	while (i--) {
 		BUG_ON(!PageSlab(page));
-		__ClearPageSlabPfmemalloc(page);
 		__ClearPageSlab(page);
 		page++;
 	}

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