linux-kernel - Re: [patch] mm, thp: always direct reclaim for MADV

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Date:   Tue, 27 Dec 2016 05:32:08 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, thp: always direct reclaim for MADV_HUGEPAGE even
 when deferred

On Mon, Dec 26, 2016 at 04:53:39PM -0800, David Rientjes wrote:
> > If there is really a need for an immediate solution^Wworkaround then I
> > think that tweaking the madvise option should be reasonably safe. Admins
> > are really prepared for stalls because they are explicitly opting in for
> > madvise behavior and they will get a background compaction on top. This
> > is a new behavior but I do not see how it would be harmful. If an
> > excessive compaction is a problem then THP can be reduced to madvise
> > only vmas.
> > 
> > But, I really _do_ care about having a stall free option which is not a
> > complete disable of the background compaction for THP.
> > 
> 
> This is completely wrong.  Before the "defer" option has been introduced, 
> we had "madvise" and should maintain its behavior as much as possible so 
> there are no surprises.  We don't change behavior for a tunable out from 
> under existing users because you think you know better.  With the new 
> "defer" option, we can make this a stronger variant of "madvise", which 
> Kirill acked, so that existing users of MADV_HUGEPAGE have no change in 
> behavior and we can configure whether we do direct or background 
> compaction for everybody else.  If people don't want background 
> compaction, they can set defrag to "madvise".  If they want it, they can 
> set it to "defer".  It's very simple.
> 
> That said, I simply don't have the time to continue in circular arguments 
> and would respectfully ask Andrew to apply this acked patch.

+1.

I don't see a point to make "defer" weaker than "madvise". MADV_HUGEPAGE
is a way for an application to say that it's okay with paying price for
huge page allocation.

-- 
 Kirill A. Shutemov