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Message-ID: <55DDC23F.8020004@suse.cz>
Date: Wed, 26 Aug 2015 15:42:23 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Mel Gorman <mgorman@...hsingularity.net>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>,
Rik van Riel <riel@...hat.com>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Michal Hocko <mhocko@...nel.org>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 12/12] mm, page_alloc: Only enforce watermarks for order-0
allocations
On 08/24/2015 02:30 PM, 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).
> 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@...hsingularity.net>
> ---
> mm/page_alloc.c | 38 ++++++++++++++++++++++++--------------
> 1 file changed, 24 insertions(+), 14 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2415f882b89c..35dc578730d1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2280,8 +2280,10 @@ static inline bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
> #endif /* CONFIG_FAIL_PAGE_ALLOC */
>
> /*
> - * Return true if free pages are above 'mark'. This takes into account the order
> - * of the allocation.
> + * Return true if free base pages are above 'mark'. For high-order checks it
> + * will return true of the order-0 watermark is reached and there is at least
> + * one free page of a suitable size. Checking now avoids taking the zone lock
> + * to check in the allocation paths if no pages are free.
> */
> static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> unsigned long mark, int classzone_idx, int alloc_flags,
> @@ -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);
>
> /* free_pages may go negative - that's OK */
> free_pages -= (1 << order) - 1;
> @@ -2301,7 +2303,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> * If the caller is not atomic then discount the reserves. This will
> * over-estimate how the atomic reserve but it avoids a search
> */
> - if (likely(!(alloc_flags & ALLOC_HARDER)))
> + if (likely(!atomic))
> free_pages -= z->nr_reserved_highatomic;
> else
> min -= min / 4;
> @@ -2309,22 +2311,30 @@ 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 */
> + 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 (atomic && area->nr_free)
> + return true;
>
> - if (free_pages <= min)
> - return false;
> + for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) {
> + if (!list_empty(&area->free_list[mt]))
> + return true;
> + }
I think we really need something like this here:
#ifdef CONFIG_CMA
if (alloc_flags & ALLOC_CMA)) &&
!list_empty(&area->free_list[MIGRATE_CMA])
return true;
#endif
This is not about CMA and high-order atomic allocations being used at
the same time. This is about high-order MIGRATE_MOVABLE allocations
(that set ALLOC_CMA) failing to use MIGRATE_CMA pageblocks, which they
should be allowed to use. It's complementary to the existing free_pages
adjustment above.
Maybe there's not many high-order MIGRATE_MOVABLE allocations today, but
they might increase with the driver migration framework. So why set up
us a bomb.
> }
> - return true;
> + return false;
> }
>
> bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
>
--
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