linux-kernel - Re: [patch v2] mm, thp: add new defer+madvise defrag option

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Date:   Thu, 12 Jan 2017 09:01:00 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jonathan Corbet <corbet@....net>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [patch v2] mm, thp: add new defer+madvise defrag option

On Wed 11-01-17 08:35:27, Vlastimil Babka wrote:
> [+CC linux-api]
> 
> On 01/11/2017 01:15 AM, David Rientjes wrote:
> > There is no thp defrag option that currently allows MADV_HUGEPAGE regions 
> > to do direct compaction and reclaim while all other thp allocations simply 
> > trigger kswapd and kcompactd in the background and fail immediately.
> > 
> > The "defer" setting simply triggers background reclaim and compaction for 
> > all regions, regardless of MADV_HUGEPAGE, which makes it unusable for our 
> > userspace where MADV_HUGEPAGE is being used to indicate the application is 
> > willing to wait for work for thp memory to be available.
> > 
> > The "madvise" setting will do direct compaction and reclaim for these
> > MADV_HUGEPAGE regions, but does not trigger kswapd and kcompactd in the 
> > background for anybody else.
> > 
> > For reasonable usage, there needs to be a mesh between the two options.  
> > This patch introduces a fifth mode, "defer+madvise", that will do direct 
> > reclaim and compaction for MADV_HUGEPAGE regions and trigger background 
> > reclaim and compaction for everybody else so that hugepages may be 
> > available in the near future.
> > 
> > A proposal to allow direct reclaim and compaction for MADV_HUGEPAGE 
> > regions as part of the "defer" mode, making it a very powerful setting and 
> > avoids breaking userspace, was offered: 
> > http://marc.info/?t=148236612700003.  This additional mode is a 
> > compromise.
> > 
> > A second proposal to allow both "defer" and "madvise" to be selected at
> > the same time was also offered: http://marc.info/?t=148357345300001.
> > This is possible, but there was a concern that it might break existing
> > userspaces the parse the output of the defrag mode, so the fifth option
> > was introduced instead.
> > 
> > This patch also cleans up the helper function for storing to "enabled" 
> > and "defrag" since the former supports three modes while the latter 
> > supports five and triple_flag_store() was getting unnecessarily messy.
> > 
> > Signed-off-by: David Rientjes <rientjes@...gle.com>
> 
> alloc_hugepage_direct_gfpmask() would have been IMHO simpler if a new
> internal flag wasn't added, and combination of two existing for defer
> and madvise used,

I agree with Vlastimil here. The patch can do without touching anything
outside of the sysfs handling.
-- 
Michal Hocko
SUSE Labs