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Subject: Re: [PATCH 1/2] mm,thp,shmem: limit shmem THP alloc gfp_mask
From:   Rik van Riel <riel@surriel.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     hughd@google.com, xuyu@linux.alibaba.com,
        akpm@linux-foundation.org, mgorman@suse.de, aarcange@redhat.com,
        willy@infradead.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, linux-mm@kvack.org, vbabka@suse.cz
Date:   Fri, 13 Nov 2020 22:44:20 -0500
In-Reply-To: <20201112105258.GZ12240@dhcp22.suse.cz>
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         <20201105191508.1961686-2-riel@surriel.com>
         <20201112105258.GZ12240@dhcp22.suse.cz>
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On Thu, 2020-11-12 at 11:52 +0100, Michal Hocko wrote:
> On Thu 05-11-20 14:15:07, Rik van Riel wrote:
> >=20
> > This patch applies the same configurated limitation of THPs to
> > shmem
> > hugepage allocations, to prevent that from happening.
>=20
> 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.
> >=20
> > 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.
>=20
> 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=3Dalways or
huge=3Dwithin_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.

--=20
All Rights Reversed.

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