linux-kernel - Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

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Message-ID: <74f2341a-7834-3e37-0346-7fbc48d74df3@suse.cz>
Date:   Wed, 2 Sep 2020 16:55:05 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pavel Tatashin <pasha.tatashin@...een.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

On 9/2/20 4:26 PM, Pavel Tatashin wrote:
> On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko <mhocko@...e.com> wrote:
>>
>> >
>> > Thread#1 - continue
>> >          free_unref_page_commit
>> >            migratetype = get_pcppage_migratetype(page);
>> >               // get old migration type
>> >            list_add(&page->lru, &pcp->lists[migratetype]);
>> >               // add new page to already drained pcp list
>> >
>> > Thread#2
>> > Never drains pcp again, and therefore gets stuck in the loop.
>> >
>> > The fix is to try to drain per-cpu lists again after
>> > check_pages_isolated_cb() fails.
>>
>> But this means that the page is not isolated and so it could be reused
>> for something else. No?
> 
> The page is in a movable zone, has zero references, and the section is
> isolated (i.e. set_pageblock_migratetype(page, MIGRATE_ISOLATE);) is
> set. The page should be offlinable, but it is lost in a pcp list as
> that list is never drained again after the first failure to migrate
> all pages in the range.

Yeah. To answer Michal's "it could be reused for something else" - yes, somebody
could allocate it from the pcplist before we do the extra drain. But then it
becomes "visible again" and the loop in __offline_pages() should catch it by
scan_movable_pages() - do_migrate_range(). And this time the pageblock is
already marked as isolated, so the page (freed by migration) won't end up on the
pcplist again.