linux-kernel - Re: [RFC PATCH 0/3] THP eligibility reporting via proc

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Message-ID: <20181207105554.GX1286@dhcp22.suse.cz>
Date:   Fri, 7 Dec 2018 11:55:54 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     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>,
        Dan Williams <dan.j.williams@...el.com>,
        David Rientjes <rientjes@...gle.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH 0/3] THP eligibility reporting via proc

On Tue 20-11-18 11:35:12, Michal Hocko wrote:
> Hi,
> this series of three patches aims at making THP eligibility reporting
> much more robust and long term sustainable. The trigger for the change
> is a regression report [1] and the long follow up discussion. In short
> the specific application didn't have good API to query whether a particular
> mapping can be backed by THP so it has used VMA flags to workaround that.
> These flags represent a deep internal state of VMAs and as such they should
> be used by userspace with a great deal of caution.
> 
> A similar has happened for [2] when users complained that VM_MIXEDMAP is
> no longer set on DAX mappings. Again a lack of a proper API led to an
> abuse.
> 
> The first patch in the series tries to emphasise that that the semantic
> of flags might change and any application consuming those should be really
> careful.
> 
> The remaining two patches provide a more suitable interface to address [1]
> and provide a consistent API to query the THP status both for each VMA
> and process wide as well.

Are there any other comments on these? I haven't heard any pushback so
far so I will re-send with RFC dropped early next week.

> 
> [1] http://lkml.kernel.org/r/http://lkml.kernel.org/r/alpine.DEB.2.21.1809241054050.224429@chino.kir.corp.google.com
> [2] http://lkml.kernel.org/r/20181002100531.GC4135@quack2.suse.cz
> 

-- 
Michal Hocko
SUSE Labs