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SubjectRe: [PATCH v2] mm/page_alloc: call check_new_pages() while zone spinlock is not held
On Tue, Mar 08, 2022 at 03:49:48PM -0800, Eric Dumazet wrote:
> On Mon, Mar 7, 2022 at 1:15 AM Mel Gorman <mgorman@techsingularity.net> wrote:
> >
> > On Fri, Mar 04, 2022 at 09:02:15AM -0800, Eric Dumazet wrote:
> > > From: Eric Dumazet <edumazet@google.com>
> > >
> > > For high order pages not using pcp, rmqueue() is currently calling
> > > the costly check_new_pages() while zone spinlock is held,
> > > and hard irqs masked.
> > >
> > > This is not needed, we can release the spinlock sooner to reduce
> > > zone spinlock contention.
> > >
> > > Note that after this patch, we call __mod_zone_freepage_state()
> > > before deciding to leak the page because it is in bad state.
> > >
> > > v2: We need to keep interrupts disabled to call __mod_zone_freepage_state()
> > >
> > > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > > Cc: Mel Gorman <mgorman@techsingularity.net>
> > > Cc: Vlastimil Babka <vbabka@suse.cz>
> > > Cc: Michal Hocko <mhocko@kernel.org>
> > > Cc: Shakeel Butt <shakeelb@google.com>
> > > Cc: Wei Xu <weixugc@google.com>
> > > Cc: Greg Thelen <gthelen@google.com>
> > > Cc: Hugh Dickins <hughd@google.com>
> > > Cc: David Rientjes <rientjes@google.com>
> >
> > Ok, this is only more expensive in the event pages on the free list have
> > been corrupted whch is already very unlikely so thanks!
> >
> > Acked-by: Mel Gorman <mgorman@techsingularity.net>
> >
>
> One remaining question is:
>
> After your patch ("mm/page_alloc: allow high-order pages to be stored
> on the per-cpu lists"),
> do we want to change check_pcp_refill()/check_new_pcp() to check all pages,
> and not only the head ?
>

We should because it was an oversight. Thanks for pointing that out.

> Or was it a conscious choice of yours ?
> (I presume part of the performance gains came from
> not having to bring ~7 cache lines per 32KB chunk on x86)
>

There will be a performance penalty due to the check but it's a correctness
vs performance issue.

This? It's boot tested only.

--8<--
mm/page_alloc: check high-order pages for corruption during PCP operations

Eric Dumazet pointed out that commit 44042b449872 ("mm/page_alloc: allow
high-order pages to be stored on the per-cpu lists") only checks the head
page during PCP refill and allocation operations. This was an oversight
and all pages should be checked. This will incur a small performance
penalty but it's necessary for correctness.

Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/page_alloc.c | 46 +++++++++++++++++++++++-----------------------
1 file changed, 23 insertions(+), 23 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3589febc6d31..2920344fa887 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2342,23 +2342,36 @@ static inline int check_new_page(struct page *page)
return 1;
}

+static bool check_new_pages(struct page *page, unsigned int order)
+{
+ int i;
+ for (i = 0; i < (1 << order); i++) {
+ struct page *p = page + i;
+
+ if (unlikely(check_new_page(p)))
+ return true;
+ }
+
+ return false;
+}
+
#ifdef CONFIG_DEBUG_VM
/*
* With DEBUG_VM enabled, order-0 pages are checked for expected state when
* being allocated from pcp lists. With debug_pagealloc also enabled, they are
* also checked when pcp lists are refilled from the free lists.
*/
-static inline bool check_pcp_refill(struct page *page)
+static inline bool check_pcp_refill(struct page *page, unsigned int order)
{
if (debug_pagealloc_enabled_static())
- return check_new_page(page);
+ return check_new_pages(page, order);
else
return false;
}

-static inline bool check_new_pcp(struct page *page)
+static inline bool check_new_pcp(struct page *page, unsigned int order)
{
- return check_new_page(page);
+ return check_new_pages(page, order);
}
#else
/*
@@ -2366,32 +2379,19 @@ static inline bool check_new_pcp(struct page *page)
* when pcp lists are being refilled from the free lists. With debug_pagealloc
* enabled, they are also checked when being allocated from the pcp lists.
*/
-static inline bool check_pcp_refill(struct page *page)
+static inline bool check_pcp_refill(struct page *page, unsigned int order)
{
- return check_new_page(page);
+ return check_new_pages(page, order);
}
-static inline bool check_new_pcp(struct page *page)
+static inline bool check_new_pcp(struct page *page, unsigned int order)
{
if (debug_pagealloc_enabled_static())
- return check_new_page(page);
+ return check_new_pages(page, order);
else
return false;
}
#endif /* CONFIG_DEBUG_VM */

-static bool check_new_pages(struct page *page, unsigned int order)
-{
- int i;
- for (i = 0; i < (1 << order); i++) {
- struct page *p = page + i;
-
- if (unlikely(check_new_page(p)))
- return true;
- }
-
- return false;
-}
-
inline void post_alloc_hook(struct page *page, unsigned int order,
gfp_t gfp_flags)
{
@@ -3037,7 +3037,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
if (unlikely(page == NULL))
break;

- if (unlikely(check_pcp_refill(page)))
+ if (unlikely(check_pcp_refill(page, order)))
continue;

/*
@@ -3641,7 +3641,7 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
page = list_first_entry(list, struct page, lru);
list_del(&page->lru);
pcp->count -= 1 << order;
- } while (check_new_pcp(page));
+ } while (check_new_pcp(page, order));

return page;
}
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