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From: Oscar Salvador <>
To: Vlastimil Babka <>
	Michal Hocko <>,
	Pavel Tatashin <>,
	David Hildenbrand <>,
	Joonsoo Kim <>,
	Michal Hocko <>
Subject: Re: [PATCH v2 7/7] mm, page_alloc: disable pcplists during memory offline
Date: Thu, 22 Oct 2020 14:52:00 +0200	[thread overview]
Message-ID: <20201022125159.GE26121@linux> (raw)
In-Reply-To: <>

On Thu, Oct 08, 2020 at 01:42:01PM +0200, Vlastimil Babka wrote:
> Memory offline relies on page isolation can race with process freeing pages to
> pcplists in a way that a page from isolated pageblock can end up on pcplist.
> 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().
> As we have to avoid external updates to high and batch while pcplists are
> disabled, we take pcp_batch_high_lock in zone_pcp_disable() and release it in
> zone_pcp_enable(). This also synchronizes multiple users of
> zone_pcp_disable()/enable().
> Currently the only user of this functionality is offline_pages().
> Suggested-by: David Hildenbrand <>
> Suggested-by: Michal Hocko <>
> Signed-off-by: Vlastimil Babka <>

I definitely like this one, and the implemantion looks smoth.
As Michal said in another thread, Hwposion code will also benefit from this,
since now we have a drain_all_pages dance that might be suboptimal and not
I will get back to that once this gets merged.

Reviewed-by: Oscar Salvador <>


Oscar Salvador

      parent reply	other threads:[~2020-10-22 12:52 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-08 11:41 [PATCH v2 0/7] " Vlastimil Babka
2020-10-08 11:41 ` [PATCH v2 1/7] mm, page_alloc: clean up pageset high and batch update Vlastimil Babka
2020-10-25 14:17   ` Mike Rapoport
2020-10-08 11:41 ` [PATCH v2 2/7] mm, page_alloc: calculate pageset high and batch once per zone Vlastimil Babka
2020-10-08 11:41 ` [PATCH v2 3/7] mm, page_alloc: remove setup_pageset() Vlastimil Babka
2020-10-08 12:23   ` Michal Hocko
2020-10-08 12:56     ` Vlastimil Babka
2020-10-08 13:03       ` Michal Hocko
2020-10-22 12:34   ` Oscar Salvador
2020-10-08 11:41 ` [PATCH v2 4/7] mm, page_alloc: simplify pageset_update() Vlastimil Babka
2020-10-22 12:39   ` Oscar Salvador
2020-10-08 11:41 ` [PATCH v2 5/7] mm, page_alloc: cache pageset high and batch in struct zone Vlastimil Babka
2020-10-08 12:31   ` Michal Hocko
2020-10-08 17:55     ` Vlastimil Babka
2020-10-22 12:42       ` Oscar Salvador
2020-10-08 11:42 ` [PATCH v2 6/7] mm, page_alloc: move draining pcplists to page isolation users Vlastimil Babka
2020-10-22 12:44   ` Oscar Salvador
2020-10-08 11:42 ` [PATCH v2 7/7] mm, page_alloc: disable pcplists during memory offline Vlastimil Babka
2020-10-08 12:45   ` Michal Hocko
2020-10-08 17:46     ` Vlastimil Babka
2020-10-22 12:52   ` Oscar Salvador [this message]

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