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

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Message-ID: <CA+CK2bChfLLDikSzaD0kCkchxiRd0fzpyDEEpbzEh_7MEqhNgw@mail.gmail.com>
Date:   Wed, 2 Sep 2020 10:31:48 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

> > > The fix is to try to drain per-cpu lists again after
> > > check_pages_isolated_cb() fails.
>
> Still trying to wrap my head around this but I think this is not a
> proper fix. It should be the page isolation to make sure no races are
> possible with the page freeing path.
>

As Bharata B Rao found in another thread, the problem was introduced
by this change:
c52e75935f8d: mm: remove extra drain pages on pcp list

So, the drain used to be tried every time with lru_add_drain_all();
Which, I think is excessive, as we start a thread per cpu to try to
drain and catch a rare race condition. With the proposed change we
drain again only when we find such a condition. Fixing it in
start_isolate_page_range means that we must somehow synchronize it
with the release_pages() which adds costs to runtime code, instead of
to hot-remove code.

Pasha