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SubjectRe: [PATCH v3] mm/page_alloc: add a helper function to check page before alloc/free
On Wed, Sep 09, 2015 at 03:39:54PM +0200, Vlastimil Babka wrote:
> On 09/08/2015 09:19 PM, Vlastimil Babka wrote:
> >bloat-o-meter looks favorably with my gcc, although there shouldn't be a real
> >reason for it, as the inlining didn't change:
> >
> >add/remove: 1/1 grow/shrink: 1/1 up/down: 285/-336 (-51)
> >function old new delta
> >bad_page - 276 +276
> >get_page_from_freelist 2521 2530 +9
> >free_pages_prepare 745 667 -78
> >bad_page.part 258 - -258
> >
> >With that,
> >
> >Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
> BTW, why do we do all these checks in non-DEBUG_VM builds? Are they
> so often hit nowadays? Shouldn't we check just for hwpoison in the
> non-debug case?

I personly think these checks are still needed in non-debug scenario so
we can still catch the bad page caused by a bug or other things in that
case.

>
> Alternatively, I've considered creating a fast inline pre-check that
> calls a non-inline check-with-report:
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0c9c82a..cff92f8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -707,7 +707,20 @@ static inline void __free_one_page(struct page *page,
> zone->free_area[order].nr_free++;
> }
>
> -static inline int check_one_page(struct page *page, unsigned long
> bad_flags)
> +static inline int check_one_page_fast(struct page *page, unsigned long
> + bad_flags)
> +{
> + return (page_mapcount(page)
> + || page->mapping != NULL
> + || atomic_read(&page->_count) != 0
> + || page->flags & bad_flags
> +#ifdef CONFIG_MEMCG
> + || page->mem_cgroup
> +#endif
> + );
> +}
> +
> +static noinline int check_one_page(struct page *page, unsigned long
> bad_flags)
> {
> const char *bad_reason = NULL;
>
> @@ -743,9 +756,12 @@ static inline int free_pages_check(struct page *page)
> {
> int ret = 0;
>
> - ret = check_one_page(page, PAGE_FLAGS_CHECK_AT_FREE);
> - if (ret)
> - return ret;
> + ret = check_one_page_fast(page, PAGE_FLAGS_CHECK_AT_FREE);
> + if (ret) {
> + ret = check_one_page(page, PAGE_FLAGS_CHECK_AT_FREE);
> + if (ret)
> + return ret;
> + }
>
> page_cpupid_reset_last(page);
> if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
> @@ -1304,7 +1320,9 @@ static inline void expand(struct zone *zone,
> struct page *page,
> */
> static inline int check_new_page(struct page *page)
> {
> - return check_one_page(page, PAGE_FLAGS_CHECK_AT_PREP);
> + if (check_one_page_fast(page, PAGE_FLAGS_CHECK_AT_PREP | __PG_HWPOISON))
> + return check_one_page(page, PAGE_FLAGS_CHECK_AT_PREP);
> + return 0;
> }
>
> static int prep_new_page(struct page *page, unsigned int order,
> gfp_t gfp_flags,
>
> ---

This looks good to me.

>
> That shrinks the fast paths nicely:
>
> add/remove: 1/1 grow/shrink: 0/2 up/down: 480/-498 (-18)
> function old new delta
> check_one_page - 480 +480
> get_page_from_freelist 2530 2458 -72
> free_pages_prepare 667 517 -150
> bad_page 276 - -276
>
> On top of that, the number of branches in the fast paths can be
> reduced if we use arithmetic OR to avoid the short-circuit boolean
> evaluation:
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cff92f8..e8b42ba 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -710,12 +710,12 @@ static inline void __free_one_page(struct page *page,
> static inline int check_one_page_fast(struct page *page, unsigned long
> bad_flags)
> {
> - return (page_mapcount(page)
> - || page->mapping != NULL
> - || atomic_read(&page->_count) != 0
> - || page->flags & bad_flags
> + return ((unsigned long) page_mapcount(page)
> + | (unsigned long) page->mapping
> + | (unsigned long) atomic_read(&page->_count)
> + | (page->flags & bad_flags)
> #ifdef CONFIG_MEMCG
> - || page->mem_cgroup
> + | (unsigned long) page->mem_cgroup
> #endif
> );
> }
>
> That further reduces the fast paths, not much in bytes, but
> importantly in branches:
>
> add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-51 (-51)
> function old new delta
> get_page_from_freelist 2458 2443 -15
> free_pages_prepare 517 481 -36
>
> But I can understand it's rather hackish, and maybe some
> architectures won't be happy with the extra unsigned long
> arithmetics. Thoughts?



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