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Subject: Re: [PATCH] mm/vmstat: Reduce zone lock hold time when reading
 /proc/pagetypeinfo
From:   Qian Cai <cai@lca.pw>
To:     Rafael Aquini <aquini@redhat.com>
Cc:     Waiman Long <longman@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Jann Horn <jannh@google.com>, Song Liu <songliubraving@fb.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date:   Wed, 23 Oct 2019 11:51:44 -0400
In-Reply-To: <20191023150319.GD22601@optiplex-lnx>
References: <20191022162156.17316-1-longman@redhat.com>
         <20191022145902.d9c4a719c0b32175e06e4eee@linux-foundation.org>
         <2236495a-ead0-e08e-3fb6-f3ab906b75b6@redhat.com>
         <1571842093.5937.84.camel@lca.pw> <20191023150319.GD22601@optiplex-lnx>
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On Wed, 2019-10-23 at 11:03 -0400, Rafael Aquini wrote:
> > > > this still isn't a bulletproof fix.  Maybe just terminate the list
> > > > walk if freecount reaches 1024.  Would anyone really care?
> > > > 
> > > > Sigh.  I wonder if anyone really uses this thing for anything
> > > > important.  Can we just remove it all?
> > > > 
> > > 
> > > Removing it will be a breakage of kernel API.
> > 
> > Who cares about breaking this part of the API that essentially nobody will use
> > this file?
> > 
> 
> Seems that _some_ are using it, aren't they? Otherwise we wouldn't be
> seeing complaints. Moving it out of /proc to /sys/kernel/debug looks 
> like a reasonable compromise with those that care about the interface.

No, there are some known testing tools will blindly read this file just because
it exists which is not important to keep the file.