From: Johannes Weiner <hannes@cmpxchg.org>
To: Shakeel Butt <shakeelb@google.com>
Cc: Hugh Dickins <hughd@google.com>,
Jerome Glisse <jglisse@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/rmap: always do TTU_IGNORE_ACCESS
Date: Thu, 5 Nov 2020 11:01:35 -0500 [thread overview]
Message-ID: <20201105160135.GF744831@cmpxchg.org> (raw)
In-Reply-To: <20201104231928.1494083-1-shakeelb@google.com>
On Wed, Nov 04, 2020 at 03:19:28PM -0800, Shakeel Butt wrote:
> Since the commit 369ea8242c0f ("mm/rmap: update to new mmu_notifier
> semantic v2"), the code to check the secondary MMU's page table access
> bit is broken for !(TTU_IGNORE_ACCESS) because the page is unmapped from
> the secondary MMU's page table before the check. More specifically for
> those secondary MMUs which unmap the memory in
> mmu_notifier_invalidate_range_start() like kvm.
>
> However memory reclaim is the only user of !(TTU_IGNORE_ACCESS) or the
> absence of TTU_IGNORE_ACCESS and it explicitly performs the page table
> access check before trying to unmap the page. So, at worst the reclaim
> will miss accesses in a very short window if we remove page table access
> check in unmapping code.
We also miss accesses that happen right after the unmap :-) Seems
completely fine to make page_referenced() the time of last call.
> There is an unintented consequence of !(TTU_IGNORE_ACCESS) for the memcg
> reclaim. From memcg reclaim the page_referenced() only account the
> accesses from the processes which are in the same memcg of the target
> page but the unmapping code is considering accesses from all the
> processes, so, decreasing the effectiveness of memcg reclaim.
>
> The simplest solution is to always assume TTU_IGNORE_ACCESS in unmapping
> code.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
next prev parent reply other threads:[~2020-11-05 16:03 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-04 23:19 [PATCH] mm/rmap: always do TTU_IGNORE_ACCESS Shakeel Butt
2020-11-05 16:01 ` Johannes Weiner [this message]
2020-11-06 3:00 ` Hugh Dickins
2020-11-06 15:09 ` Shakeel Butt
2020-11-11 7:50 ` Hugh Dickins
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