Messages in this thread Patches in this message | | | Subject | Re: [PATCH v3] mm/page_alloc: add a helper function to check page before alloc/free | From | Vlastimil Babka <> | Date | Wed, 9 Sep 2015 15:39:54 +0200 |
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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?
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, --- 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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