Messages in this thread Patch in this message | | | From | Vlastimil Babka <> | Subject | [PATCH] mm, page_alloc: fix potential false positive in __zone_watermark_ok | Date | Thu, 2 Nov 2017 13:50:01 +0100 |
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Since commit 97a16fc82a7c ("mm, page_alloc: only enforce watermarks for order-0 allocations"), __zone_watermark_ok() check for high-order allocations will shortcut per-migratetype free list checks for ALLOC_HARDER allocations, and return true as long as there's free page of any migratetype. The intention is that ALLOC_HARDER can allocate from MIGRATE_HIGHATOMIC free lists, while normal allocations can't.
However, as a side effect, the watermark check will then also return true when there are pages only on the MIGRATE_ISOLATE list, or (prior to CMA conversion to ZONE_MOVABLE) on the MIGRATE_CMA list. Since the allocation cannot actually obtain isolated pages, and might not be able to obtain CMA pages, this can result in a false positive.
The condition should be rare and perhaps the outcome is not a fatal one. Still, it's better if the watermark check is correct. There also shouldn't be a performance tradeoff here.
Fixes: 97a16fc82a7c ("mm, page_alloc: only enforce watermarks for order-0 allocations") Signed-off-by: Vlastimil Babka <vbabka@suse.cz> --- mm/page_alloc.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 79cdac1fee42..f43039945148 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3032,9 +3032,6 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark, if (!area->nr_free) continue; - if (alloc_harder) - return true; - for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) { if (!list_empty(&area->free_list[mt])) return true; @@ -3044,6 +3041,9 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark, if (!list_empty(&area->free_list[MIGRATE_CMA])) return true; #endif + if (alloc_harder && + !list_empty(&area->free_list[MIGRATE_HIGHATOMIC])) + return true; } return false; } -- 2.14.3
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