linux-kernel - Re: [PATCH 1/2] mm,thp,shmem: limit shmem THP alloc gfp

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Date:   Fri, 13 Nov 2020 22:44:20 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     hughd@...gle.com, xuyu@...ux.alibaba.com,
        akpm@...ux-foundation.org, mgorman@...e.de, aarcange@...hat.com,
        willy@...radead.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-mm@...ck.org, vbabka@...e.cz
Subject: Re: [PATCH 1/2] mm,thp,shmem: limit shmem THP alloc gfp_mask

On Thu, 2020-11-12 at 11:52 +0100, Michal Hocko wrote:
> On Thu 05-11-20 14:15:07, Rik van Riel wrote:
> > 
> > This patch applies the same configurated limitation of THPs to
> > shmem
> > hugepage allocations, to prevent that from happening.
> 
> I believe you should also exaplain why we want to control defrag by
> the
> global knob while the enable logic is per mount.

I added that to the changelog for the next version of
the patches.

> > This way a THP defrag setting of "never" or "defer+madvise" will
> > result
> > in quick allocation failures without direct reclaim when no 2MB
> > free
> > pages are available.
> > 
> > With this patch applied, THP allocations for tmpfs will be a little
> > more aggressive than today for files mmapped with MADV_HUGEPAGE,
> > and a little less aggressive for files that are not mmapped or
> > mapped without that flag.
> 
> This begs some numbers. A little is rather bad unit of performance. I
> do
> agree that unifying those makes sense in general though.

The aggressiveness is in changes to the gfp_mask, eg by
adding __GFP_NORETRY. How that translates into THP
allocation success rates is entirely dependent on the
workload and on what else is in memory at the time.

I am not sure any
numbers I could gather will be
representative for anything but the workloads I am
testing.

However, I did find an issue in hugepage_vma_check
that prevents khugepaged from collapsing pages on
shmem filesystems mounted with huge=always or
huge=within_size when transparent_hugepage/enabled
is set to [madvise].

The next version of the series will have a third
patch, in order to fix that.

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