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Date: Wed, 30 Sep 2015 16:02:19 +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 09/10] mm, page_alloc: Reserve pageblocks for high-order
atomic allocations on demand
On 09/30/2015 10:27 AM, Mel Gorman wrote:
> On Tue, Sep 29, 2015 at 02:01:41PM -0700, Andrew Morton wrote:
>>> ...
>>>
>>> +/*
>>> + * Reserve a pageblock for exclusive use of high-order atomic allocations if
>>> + * there are no empty page blocks that contain a page with a suitable order
>>> + */
>>> +static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
>>> + unsigned int alloc_order)
>>> +{
>>> + int mt;
>>> + unsigned long max_managed, flags;
>>> +
>>> + /*
>>> + * Limit the number reserved to 1 pageblock or roughly 1% of a zone.
>>> + * Check is race-prone but harmless.
>>> + */
>>> + max_managed = (zone->managed_pages / 100) + pageblock_nr_pages;
>>> + if (zone->nr_reserved_highatomic >= max_managed)
>>> + return;
>>> +
>>> + /* Yoink! */
>>> + spin_lock_irqsave(&zone->lock, flags);
>>> +
>>> + mt = get_pageblock_migratetype(page);
>>> + if (mt != MIGRATE_HIGHATOMIC &&
>>> + !is_migrate_isolate(mt) && !is_migrate_cma(mt)) {
>>
>> Do the above checks really need to be inside zone->lock? I don't think
>> get_pageblock_migratetype() needs zone->lock? (Actually I suspect it
>> does, but we don't...)
>>
>
> The get_pageblock_migratetype does not require zone->lock but it's race-prone
> without it and there have been cases (CMA, isolation) that cared. In this
> case, without the lock two parallel allocations may try to reserve the same
> block so we'd have to recheck the type under the lock to avoid corrupting
> nr_reserved_highatomic. As the move between free lists absolutely requires
> the zone->lock, it's best to just do the full operation under the lock.
>
>>> + zone->nr_reserved_highatomic += pageblock_nr_pages;
>>
>> And I don't think it would hurt to recheck
>> nr_reserved_highatomic>=max_managed after taking zone->lock, to plug
>> that race. We've had VM we-dont-care races in the past which ended up
>> causing problems in rare circumstances...
>>
>
> That makes sense, patch is below.
>
>>> + set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
>>> + move_freepages_block(zone, page, MIGRATE_HIGHATOMIC);
>>> + }
>>> + spin_unlock_irqrestore(&zone->lock, flags);
>>> +}
>>> +
>>> +/*
>>> + * Used when an allocation is about to fail under memory pressure. This
>>> + * potentially hurts the reliability of high-order allocations when under
>>> + * intense memory pressure but failed atomic allocations should be easier
>>> + * to recover from than an OOM.
>>> + */
>>> +static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
>>> +{
>>> + struct zonelist *zonelist = ac->zonelist;
>>> + unsigned long flags;
>>> + struct zoneref *z;
>>> + struct zone *zone;
>>> + struct page *page;
>>> + int order;
>>> +
>>> + for_each_zone_zonelist_nodemask(zone, z, zonelist, ac->high_zoneidx,
>>> + ac->nodemask) {
>>> + /* Preserve at least one pageblock */
>>> + if (zone->nr_reserved_highatomic <= pageblock_nr_pages)
>>> + continue;
>>> +
>>> + spin_lock_irqsave(&zone->lock, flags);
>>> + for (order = 0; order < MAX_ORDER; order++) {
>>> + struct free_area *area = &(zone->free_area[order]);
>>> +
>>> + if (list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
>>> + continue;
>>> +
>>> + page = list_entry(area->free_list[MIGRATE_HIGHATOMIC].next,
>>> + struct page, lru);
>>> +
>>> + zone->nr_reserved_highatomic -= pageblock_nr_pages;
>>
>> So if the race happened here, zone->nr_reserved_highatomic underflows?
>>
>
> It shouldn't. If there are entries on the MIGRATE_HIGHATOMIC list then
> it should be accounted for in nr_reserved_highatomic. However, I see your
> point as a spill from per-cpu lists has caused us problems in the past.
>
> ---8<---
> From: Mel Gorman <mgorman@...hsingularity.net>
> Subject: [PATCH] mm, page_alloc: Reserve pageblocks for high-order atomic
> allocations on demand -fix
>
> nr_reserved_highatomic is checked outside the zone lock so there is a race
> whereby the reserve is larger than the limit allows. This patch rechecks
> the count under the zone lock.
>
> During unreserving, there is a possibility we could underflow if there
> ever was a race between per-cpu drains, reserve and unreserving. This
> patch adds a comment about the potential race and protects against it.
>
> These are two fixes to the mmotm patch
> mm-page_alloc-reserve-pageblocks-for-high-order-atomic-allocations-on-demand.patch .
> They are not separate patches and they should all be folded together.
>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
Ack.
> ---
> mm/page_alloc.c | 17 +++++++++++++++--
> 1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 811d6fc4ad5d..b1892dc51b55 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1633,9 +1633,13 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
> if (zone->nr_reserved_highatomic >= max_managed)
> return;
>
> - /* Yoink! */
> spin_lock_irqsave(&zone->lock, flags);
>
> + /* Recheck the nr_reserved_highatomic limit under the lock */
> + if (zone->nr_reserved_highatomic >= max_managed)
> + goto out_unlock;
> +
> + /* Yoink! */
> mt = get_pageblock_migratetype(page);
> if (mt != MIGRATE_HIGHATOMIC &&
> !is_migrate_isolate(mt) && !is_migrate_cma(mt)) {
> @@ -1643,6 +1647,8 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
> set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
> move_freepages_block(zone, page, MIGRATE_HIGHATOMIC);
> }
> +
> +out_unlock:
> spin_unlock_irqrestore(&zone->lock, flags);
> }
>
> @@ -1677,7 +1683,14 @@ static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
> page = list_entry(area->free_list[MIGRATE_HIGHATOMIC].next,
> struct page, lru);
>
> - zone->nr_reserved_highatomic -= pageblock_nr_pages;
> + /*
> + * It should never happen but changes to locking could
> + * inadvertently allow a per-cpu drain to add pages
> + * to MIGRATE_HIGHATOMIC while unreserving so be safe
> + * and watch for underflows.
> + */
> + zone->nr_reserved_highatomic -= min(pageblock_nr_pages,
> + zone->nr_reserved_highatomic);
>
> /*
> * Convert to ac->migratetype and avoid the normal
>
--
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