linux-kernel - Re: [RFC PATCH 3/3] mm, proc: report PR_SET_THP

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Date:   Fri, 23 Nov 2018 16:49:04 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>, linux-api@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH 3/3] mm, proc: report PR_SET_THP_DISABLE in proc

On 11/20/18 11:35 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> David Rientjes has reported that 1860033237d4 ("mm: make
> PR_SET_THP_DISABLE immediately active") has changed the way how
> we report THPable VMAs to the userspace. Their monitoring tool is
> triggering false alarms on PR_SET_THP_DISABLE tasks because it considers
> an insufficient THP usage as a memory fragmentation resp. memory
> pressure issue.
> 
> Before the said commit each newly created VMA inherited VM_NOHUGEPAGE
> flag and that got exposed to the userspace via /proc/<pid>/smaps file.
> This implementation had its downsides as explained in the commit message
> but it is true that the userspace doesn't have any means to query for
> the process wide THP enabled/disabled status.
> 
> PR_SET_THP_DISABLE is a process wide flag so it makes a lot of sense
> to export in the process wide context rather than per-vma. Introduce

Agreed.

> a new field to /proc/<pid>/status which export this status.  If
> PR_SET_THP_DISABLE is used then it reports false same as when the THP is
> not compiled in. It doesn't consider the global THP status because we
> already export that information via sysfs
> 
> Fixes: 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
> Signed-off-by: Michal Hocko <mhocko@...e.com>

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