On Mon 26-09-16 17:16:54, Xishi Qiu wrote: > On 2016/9/26 16:58, Michal Hocko wrote: > > > On Mon 26-09-16 16:47:57, Xishi Qiu wrote: > >> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1 > >> (mm, page_alloc: only enforce watermarks for order-0 allocations) > >> rewrite the high-order check in __zone_watermark_ok(), but I think it > >> quietly fix a bug. Please see the following. > >> > >> Before this patch, the high-order check is this: > >> __zone_watermark_ok() > >> ... > >> for (o = 0; o < order; o++) { > >> /* At the next order, this order's pages become unavailable */ > >> free_pages -= z->free_area[o].nr_free << o; > >> > >> /* Require fewer higher order pages to be free */ > >> min >>= 1; > >> > >> if (free_pages <= min) > >> return false; > >> } > >> ... > >> > >> If we have cma memory, and we alloc a high-order movable page, then it's right. > >> > >> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()), > >> and there are a lot of high-order cma pages, but little high-order unmovable > >> pages, the it is still return *true*, but we will alloc *failed* finally, because > >> we cannot fallback from migrate_unmovable to migrate_cma, right? > > > > AFAIR CMA wmark check was always tricky and the above commit has made > > the situation at least a bit more clear. Anyway IIRC > > > > #ifdef CONFIG_CMA > > /* If allocation can't use CMA areas don't use free CMA pages */ > > if (!(alloc_flags & ALLOC_CMA)) > > free_cma = zone_page_state(z, NR_FREE_CMA_PAGES); > > #endif > > > > if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx]) > > return false; > > > > should reduce the prioblem because a lot of CMA pages should just get us > > below the wmark + reserve boundary. > > Hi Michal, > > If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable) > are also enough, but they are fragment, then it will triger the problem. > If we alloc a high-order unmovable page, water mark check return *true*, but we > will alloc *failed*, right? As Vlastimil has written. There were known issues with the wmark checks and high order requests. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>