Re: [PATCH 10/10] mm, page_alloc: Only enforce watermarks for order-0 allocations

From: Andrew Morton
Date: Tue Sep 29 2015 - 17:05:15 EST


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;
> }

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