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Date:   Thu, 26 Nov 2020 17:56:47 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Rik van Riel <riel@...riel.com>, 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 1/3] mm,thp,shmem: limit shmem THP alloc gfp_mask

On 11/24/20 8:49 PM, Rik van Riel wrote:
> The allocation flags of anonymous transparent huge pages can be controlled
> through the files in /sys/kernel/mm/transparent_hugepage/defrag, which can
> help the system from getting bogged down in the page reclaim and compaction
> code when many THPs are getting allocated simultaneously.
> 
> However, the gfp_mask for shmem THP allocations were not limited by those
> configuration settings, and some workloads ended up with all CPUs stuck
> on the LRU lock in the page reclaim code, trying to allocate dozens of
> THPs simultaneously.
> 
> This patch applies the same configurated limitation of THPs to shmem
> hugepage allocations, to prevent that from happening.
> 
> Controlling the gfp_mask of THP allocations through the knobs in
> sysfs allows users to determine the balance between how aggressively
> the system tries to allocate THPs at fault time, and how much the
> application may end up stalling attempting those allocations.
> 
> 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.
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

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