linux-kernel - Re: [RFC 0/2] Use HighAtomic against long-term fragmentation

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Date:   Tue, 26 Sep 2017 10:51:27 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Hui Zhu <zhuhui@...omi.com>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com, vbabka@...e.cz,
        hillf.zj@...baba-inc.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, teawater@...il.com
Subject: Re: [RFC 0/2] Use HighAtomic against long-term fragmentation

On Tue, Sep 26, 2017 at 04:46:42PM +0800, Hui Zhu wrote:
> Current HighAtomic just to handle the high atomic page alloc.
> But I found that use it handle the normal unmovable continuous page
> alloc will help to against long-term fragmentation.
> 

This is not wise. High-order atomic allocations do not always have a
smooth recovery path such as network drivers with large MTUs that have no
choice but to drop the traffic and hope for a retransmit. That's why they
have the highatomic reserve. If the reserve is used for normal unmovable
allocations then allocation requests that could have waited for reclaim
may cause high-order atomic allocations to fail. Changing it may allow
improve latencies in some limited cases while causing functional failures
in others.  If there is a special case where there are a large number of
other high-order allocations then I would suggest increasing min_free_kbytes
instead as a workaround.

-- 
Mel Gorman
SUSE Labs