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" <cl@xxxxxxxxx>
>
> 2012/9/5 Mel Gorman <mgorman@xxxxxxx>:
> > 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@xxxxxxxxx: Original implementation and problem identification]
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > ---
> > 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.
> > @@ -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.
> @@ -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.
> @@ -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 <js1304@xxxxxxxxx>
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@xxxxxxx: Easier implementation, changelog cleanup]
Signed-off-by: Joonsoo Kim <js1304@xxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
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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