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Subject: Re: [PATCH 3/3] mm,thp,shmem: make khugepaged obey tmpfs mount flags
From:   Rik van Riel <riel@surriel.com>
To:     Vlastimil Babka <vbabka@suse.cz>, hughd@google.com
Cc:     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, mhocko@suse.com
Date:   Thu, 26 Nov 2020 13:14:55 -0500
In-Reply-To: <eda496c8-e248-c8fe-e7bd-f7e71d20e499@suse.cz>
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         <20201124194925.623931-4-riel@surriel.com>
         <eda496c8-e248-c8fe-e7bd-f7e71d20e499@suse.cz>
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On Thu, 2020-11-26 at 18:18 +0100, Vlastimil Babka wrote:
> On 11/24/20 8:49 PM, Rik van Riel wrote:
> > Currently if thp enabled=3D[madvise], mounting a tmpfs filesystem
> > with huge=3Dalways and mmapping files from that tmpfs does not
> > result in khugepaged collapsing those mappings, despite the
> > mount flag indicating that it should.
> >=20
> > Fix that by breaking up the blocks of tests in hugepage_vma_check
> > a little bit, and testing things in the correct order.
> >=20
> > Signed-off-by: Rik van Riel <riel@surriel.com>
> > Fixes: c2231020ea7b ("mm: thp: register mm for khugepaged when
> > merging vma for shmem")
>=20
> Looks ok. But, it we have sysfs thp enabled=3Dnever, and shmem mount
> explicitly=20
> thp enabled, then shmem mount overrides the global sysfs setting and
> thp's will=20
> be allocated there, right? However, khugepaged_enabled() will be
> false and thus=20
> khugepaged won't run at all? So a similar situation than what you're
> fixing here.

Indeed, that is somewhat similar. Whether or not shmem
allocations attempt huge pages is controlled by both
the file /sys/kernel/mm/transparent_hugepage/shmem_enabled
and mount options.

This patch makes khugepaged treat the mount options
and/or
sysfs flag as enabling collapsing of huge pages in case
enabled =3D [always] for regular THPs.

Should I send another patch on top
of this that causes
khugepaged to be enabled when regular THPs are disabled,
but shmem THPs are enabled in any way?

--=20
All Rights Reversed.

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