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From: David Hildenbrand <david@redhat.com>
To: Vlastimil Babka <vbabka@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Michal Hocko <mhocko@kernel.org>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	Oscar Salvador <osalvador@suse.de>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH v3 7/7] mm, page_alloc: disable pcplists during memory offline
Date: Wed, 11 Nov 2020 18:59:38 +0100	[thread overview]
Message-ID: <3317cf76-458c-8873-0e1a-4129745f0fda@redhat.com> (raw)
In-Reply-To: <20201111092812.11329-8-vbabka@suse.cz>

On 11.11.20 10:28, Vlastimil Babka wrote:
> Memory offlining relies on page isolation to guarantee a forward
> progress because pages cannot be reused while they are isolated. But the
> page isolation itself doesn't prevent from races while freed pages are
> stored on pcp lists and thus can be reused.  This can be worked around by
> repeated draining of pcplists, as done by commit 968318261221
> ("mm/memory_hotplug: drain per-cpu pages again during memory offline").
> 
> David and Michal would prefer that this race was closed in a way that callers
> of page isolation who need stronger guarantees don't need to repeatedly drain.
> David suggested disabling pcplists usage completely during page isolation,
> instead of repeatedly draining them.
> 
> To achieve this without adding special cases in alloc/free fastpath, we can use
> the same approach as boot pagesets - when pcp->high is 0, any pcplist addition
> will be immediately flushed.
> 
> The race can thus be closed by setting pcp->high to 0 and draining pcplists
> once, before calling start_isolate_page_range(). The draining will serialize
> after processes that already disabled interrupts and read the old value of
> pcp->high in free_unref_page_commit(), and processes that have not yet disabled
> interrupts, will observe pcp->high == 0 when they are rescheduled, and skip
> pcplists. This guarantees no stray pages on pcplists in zones where isolation
> happens.
> 
> This patch thus adds zone_pcp_disable() and zone_pcp_enable() functions that
> page isolation users can call before start_isolate_page_range() and after
> unisolating (or offlining) the isolated pages.
> 
> Also, drain_all_pages() is optimized to only execute on cpus where pcplists are
> not empty. The check can however race with a free to pcplist that has not yet
> increased the pcp->count from 0 to 1. Thus make the drain optionally skip the
> racy check and drain on all cpus, and use this option in zone_pcp_disable().

^ there it is, can you move that comment into the code? People (e.g., me 
:) ) staring at that will be confused otherwise.

-- 
Thanks,

David / dhildenb


      parent reply	other threads:[~2020-11-11 18:00 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-11  9:28 [PATCH v3 0/7] " Vlastimil Babka
2020-11-11  9:28 ` [PATCH v3 1/7] mm, page_alloc: clean up pageset high and batch update Vlastimil Babka
2020-11-11  9:55   ` Pankaj Gupta
2020-11-11  9:55     ` Pankaj Gupta
2020-11-11  9:28 ` [PATCH v3 2/7] mm, page_alloc: calculate pageset high and batch once per zone Vlastimil Babka
2020-11-11 10:19   ` Pankaj Gupta
2020-11-11 10:19     ` Pankaj Gupta
2020-11-11  9:28 ` [PATCH v3 3/7] mm, page_alloc: remove setup_pageset() Vlastimil Babka
2020-11-11 10:23   ` Pankaj Gupta
2020-11-11 10:23     ` Pankaj Gupta
2020-11-11  9:28 ` [PATCH v3 4/7] mm, page_alloc: simplify pageset_update() Vlastimil Babka
2020-11-11  9:28 ` [PATCH v3 5/7] mm, page_alloc: cache pageset high and batch in struct zone Vlastimil Babka
2020-11-12 16:26   ` David Hildenbrand
2020-11-11  9:28 ` [PATCH v3 6/7] mm, page_alloc: move draining pcplists to page isolation users Vlastimil Babka
2020-11-11  9:28 ` [PATCH v3 7/7] mm, page_alloc: disable pcplists during memory offline Vlastimil Babka
2020-11-11 17:58   ` David Hildenbrand
2020-11-12 15:18     ` Vlastimil Babka
2020-11-12 16:09       ` David Hildenbrand
2020-11-11 17:59   ` David Hildenbrand [this message]

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