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Subject: Re: [RFC PATCH] mm, oom: introduce vm.sacrifice_hugepage_on_oom
To:     Eiichi Tsukata <eiichi.tsukata@nutanix.com>, corbet@lwn.net,
        mike.kravetz@oracle.com, mcgrof@kernel.org, keescook@chromium.org,
        yzaikin@google.com, akpm@linux-foundation.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org
Cc:     felipe.franciosi@nutanix.com
References: <20210216030713.79101-1-eiichi.tsukata@nutanix.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <dc18e98e-2467-bb36-7f78-d7003d9aa5f9@redhat.com>
Date:   Wed, 17 Feb 2021 10:09:58 +0100
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On 16.02.21 04:07, Eiichi Tsukata wrote:
> Hugepages can be preallocated to avoid unpredictable allocation latency.
> If we run into 4k page shortage, the kernel can trigger OOM even though
> there were free hugepages. When OOM is triggered by user address page
> fault handler, we can use oom notifier to free hugepages in user space
> but if it's triggered by memory allocation for kernel, there is no way
> to synchronously handle it in user space.
> 
> This patch introduces a new sysctl vm.sacrifice_hugepage_on_oom. If
> enabled, it first tries to free a hugepage if available before invoking
> the oom-killer. The default value is disabled not to change the current
> behavior.

In addition to the other comments, some more thoughts:

What if you're low on kernel memory but you end up freeing huge pages 
residing in ZONE_MOVABLE? IOW, this is not zone aware.

-- 
Thanks,

David / dhildenb