On Mon, 21 Sep 2015 13:03:17 +0100 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote: > The primary purpose of watermarks is to ensure that reclaim can always > make forward progress in PF_MEMALLOC context (kswapd and direct reclaim). > These assume that order-0 allocations are all that is necessary for > forward progress. > > High-order watermarks serve a different purpose. Kswapd > had no high-order awareness before they were introduced > (https://lkml.kernel.org/r/413AA7B2.4000907@xxxxxxxxxxxx). This was > particularly important when there were high-order atomic requests. > The watermarks both gave kswapd awareness and made a reserve for those > atomic requests. > > There are two important side-effects of this. The most important is that > a non-atomic high-order request can fail even though free pages are available > and the order-0 watermarks are ok. The second is that high-order watermark > checks are expensive as the free list counts up to the requested order must > be examined. > > With the introduction of MIGRATE_HIGHATOMIC it is no longer necessary to > have high-order watermarks. Kswapd and compaction still need high-order > awareness which is handled by checking that at least one suitable high-order > page is free. > > With the patch applied, there was little difference in the allocation > failure rates as the atomic reserves are small relative to the number of > allocation attempts. The expected impact is that there will never be an > allocation failure report that shows suitable pages on the free lists. > > The one potential side-effect of this is that in a vanilla kernel, the > watermark checks may have kept a free page for an atomic allocation. Now, > we are 100% relying on the HighAtomic reserves and an early allocation to > have allocated them. If the first high-order atomic allocation is after > the system is already heavily fragmented then it'll fail. > > ... > > static bool __zone_watermark_ok(struct zone *z, unsigned int order, > unsigned long mark, int classzone_idx, int alloc_flags, > @@ -2317,7 +2319,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order, > { > long min = mark; > int o; > - long free_cma = 0; > + const bool alloc_harder = (alloc_flags & ALLOC_HARDER); hmpf. Setting a bool to 0x10 is a bit grubby. > /* free_pages may go negative - that's OK */ > free_pages -= (1 << order) - 1; > @@ -2330,7 +2332,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order, > * the high-atomic reserves. This will over-estimate the size of the > * atomic reserve but it avoids a search. > */ > - if (likely(!(alloc_flags & ALLOC_HARDER))) > + if (likely(!alloc_harder)) > free_pages -= z->nr_reserved_highatomic; > else > min -= min / 4; > @@ -2338,22 +2340,43 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order, > #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); > + free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES); > #endif > > - if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx]) > + if (free_pages <= min + z->lowmem_reserve[classzone_idx]) > return false; > - 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; > + /* order-0 watermarks are ok */ because? > + if (!order) > + return true; > + > + /* Check at least one high-order page is free */ > + for (o = order; o < MAX_ORDER; o++) { > + struct free_area *area = &z->free_area[o]; > + int mt; > + > + if (!area->nr_free) > + continue; > + > + if (alloc_harder) { > + if (area->nr_free) > + return true; > + continue; > + } > > - if (free_pages <= min) > - return false; > + for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) { > + if (!list_empty(&area->free_list[mt])) > + return true; > + } > + > +#ifdef CONFIG_CMA > + if ((alloc_flags & ALLOC_CMA) && > + !list_empty(&area->free_list[MIGRATE_CMA])) { > + return true; > + } > +#endif > } > - return true; > + return false; > } -- 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>