linux-kernel - Re: [PATCH 3/3] mm,thp,shmem: make khugepaged obey tmpfs mount flags

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Date:   Thu, 26 Nov 2020 13:14:55 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Vlastimil Babka <vbabka@...e.cz>, hughd@...gle.com
Cc:     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, mhocko@...e.com
Subject: Re: [PATCH 3/3] mm,thp,shmem: make khugepaged obey tmpfs mount flags

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=[madvise], mounting a tmpfs filesystem
> > with huge=always and mmapping files from that tmpfs does not
> > result in khugepaged collapsing those mappings, despite the
> > mount flag indicating that it should.
> > 
> > Fix that by breaking up the blocks of tests in hugepage_vma_check
> > a little bit, and testing things in the correct order.
> > 
> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > Fixes: c2231020ea7b ("mm: thp: register mm for khugepaged when
> > merging vma for shmem")
> 
> Looks ok. But, it we have sysfs thp enabled=never, and shmem mount
> explicitly 
> thp enabled, then shmem mount overrides the global sysfs setting and
> thp's will 
> be allocated there, right? However, khugepaged_enabled() will be
> false and thus 
> 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 = [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?

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