On Mon 24-08-15 13:30:15, Mel Gorman 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.org/lkml/2004/9/5/9). lkml.org sucks. Could you plase replace it by something else e.g. 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. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> [...] > @@ -2289,7 +2291,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order, > { > long min = mark; > int o; > - long free_cma = 0; > + const bool atomic = (alloc_flags & ALLOC_HARDER); I just find the naming a bit confusing. ALLOC_HARDER != __GFP_ATOMIC. RT tasks might get access to this reserve as well. [...] > + /* 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 (atomic && area->nr_free) > + return true; Didn't you want if (atomic) { 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; > + } > } > - return true; > + return false; > } > > bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark, > -- > 2.4.6 -- 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>