Re: [PATCH v2 2/8] mm/page_alloc: correct to clear guard attribute in DEBUG_PAGEALLOC
From: Minchan Kim <minchan@kernel.org>
Date: 2014-08-12 01:40:53
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On Wed, Aug 06, 2014 at 04:18:30PM +0900, Joonsoo Kim wrote:
In __free_one_page(), we check the buddy page if it is guard page. And, if so, we should clear guard attribute on the buddy page. But, currently, we clear original page's order rather than buddy one's. This doesn't have any problem, because resetting buddy's order is useless and the original page's order is re-assigned soon. But, it is better to correct code. Additionally, I change (set/clear)_page_guard_flag() to (set/clear)_page_guard() and makes these functions do all works needed for guard page. This may make code more understandable. One more thing, I did in this patch, is that fixing freepage accounting. If we clear guard page and link it onto isolate buddy list, we should not increase freepage count.
You are saying just "shouldn't do that" but don't say "why" and "result" I know the reason but as you know, I'm one of the person who is rather familiar with this part but I guess others should spend some time to get. Kind detail description is never to look down on person. :)
Nice catch, Joonsoo! But what make me worry is is this patch makes 3 thing all at once. 1. fix - no candidate for stable 2. clean up 3. fix - candidate for stable. Could you separate 3 and (1,2) in next spin?
quoted hunk ↗ jump to hunk
Acked-by: Vlastimil Babka <redacted> Signed-off-by: Joonsoo Kim <redacted> --- mm/page_alloc.c | 29 ++++++++++++++++------------- 1 file changed, 16 insertions(+), 13 deletions(-)diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 44672dc..3e1e344 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c@@ -441,18 +441,28 @@ static int __init debug_guardpage_minorder_setup(char *buf) } __setup("debug_guardpage_minorder=", debug_guardpage_minorder_setup); -static inline void set_page_guard_flag(struct page *page) +static inline void set_page_guard(struct zone *zone, struct page *page, + unsigned int order, int migratetype) { __set_bit(PAGE_DEBUG_FLAG_GUARD, &page->debug_flags); + set_page_private(page, order); + /* Guard pages are not available for any usage */ + __mod_zone_freepage_state(zone, -(1 << order), migratetype); } -static inline void clear_page_guard_flag(struct page *page) +static inline void clear_page_guard(struct zone *zone, struct page *page, + unsigned int order, int migratetype) { __clear_bit(PAGE_DEBUG_FLAG_GUARD, &page->debug_flags); + set_page_private(page, 0); + if (!is_migrate_isolate(migratetype)) + __mod_zone_freepage_state(zone, (1 << order), migratetype); } #else -static inline void set_page_guard_flag(struct page *page) { } -static inline void clear_page_guard_flag(struct page *page) { } +static inline void set_page_guard(struct zone *zone, struct page *page, + unsigned int order, int migratetype) {} +static inline void clear_page_guard(struct zone *zone, struct page *page, + unsigned int order, int migratetype) {} #endif static inline void set_page_order(struct page *page, unsigned int order)@@ -594,10 +604,7 @@ static inline void __free_one_page(struct page *page, * merge with it and move up one order. */ if (page_is_guard(buddy)) { - clear_page_guard_flag(buddy); - set_page_private(page, 0); - __mod_zone_freepage_state(zone, 1 << order, - migratetype); + clear_page_guard(zone, buddy, order, migratetype); } else { list_del(&buddy->lru); zone->free_area[order].nr_free--;@@ -876,11 +883,7 @@ static inline void expand(struct zone *zone, struct page *page, * pages will stay not present in virtual address space */ INIT_LIST_HEAD(&page[size].lru); - set_page_guard_flag(&page[size]); - set_page_private(&page[size], high); - /* Guard pages are not available for any usage */ - __mod_zone_freepage_state(zone, -(1 << high), - migratetype); + set_page_guard(zone, &page[size], high, migratetype); continue; } #endif-- 1.7.9.5
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