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Date:   Tue, 12 Feb 2019 12:34:09 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Roman Gushchin <guroan@gmail.com>, linux-mm@kvack.org,
        Matthew Wilcox <willy@infradead.org>, kernel-team@fb.com,
        linux-kernel@vger.kernel.org, Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH v2 0/3] vmalloc enhancements
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In-Reply-To: <20190212184724.GA18339@cmpxchg.org>
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        <20190212184724.GA18339@cmpxchg.org>
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On Tue, 12 Feb 2019 13:47:24 -0500 Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Tue, Feb 12, 2019 at 09:56:45AM -0800, Roman Gushchin wrote:
> > The patchset contains few changes to the vmalloc code, which are
> > leading to some performance gains and code simplification.
> > 
> > Also, it exports a number of pages, used by vmalloc(),
> > in /proc/meminfo.
> > 
> > Patch (1) removes some redundancy on __vunmap().
> > Patch (2) separates memory allocation and data initialization
> >   in alloc_vmap_area()
> > Patch (3) adds vmalloc counter to /proc/meminfo.
> > 
> > v2->v1:
> >   - rebased on top of current mm tree
> >   - switch from atomic to percpu vmalloc page counter
> 
> I don't understand what prompted this change to percpu counters.
> 
> All writers already write vmap_area_lock and vmap_area_list, so it's
> not really saving much. The for_each_possible_cpu() for /proc/meminfo
> on the other hand is troublesome.

percpu_counters would fit here.  They have probably-unneeded locking
but I expect that will be acceptable.

And they address the issues with for_each_possible_cpu() avoidance, CPU
hotplug and transient negative values.