linux-kernel - Re: [PATCH 1/6] mm: change locked_vm's type from unsigned long to atomic64

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Date:   Wed, 3 Apr 2019 12:09:03 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        akpm@...ux-foundation.org, Davidlohr Bueso <dave@...olabs.net>,
        kvm@...r.kernel.org, Alan Tull <atull@...nel.org>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-mm@...ck.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Moritz Fischer <mdf@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        linuxppc-dev@...ts.ozlabs.org, Wu Hao <hao.wu@...el.com>
Subject: Re: [PATCH 1/6] mm: change locked_vm's type from unsigned long to
 atomic64_t

On Wed, Apr 03, 2019 at 06:46:07AM +0200, Christophe Leroy wrote:
> 
> 
> Le 02/04/2019 à 22:41, Daniel Jordan a écrit :
> > Taking and dropping mmap_sem to modify a single counter, locked_vm, is
> > overkill when the counter could be synchronized separately.
> > 
> > Make mmap_sem a little less coarse by changing locked_vm to an atomic,
> > the 64-bit variety to avoid issues with overflow on 32-bit systems.
> 
> Can you elaborate on the above ? Previously it was 'unsigned long', what
> were the issues ?

Sure, I responded to this in another thread from this series.

> If there was such issues, shouldn't there be a first patch
> moving it from unsigned long to u64 before this atomic64_t change ? Or at
> least it should be clearly explain here what the issues are and how
> switching to a 64 bit counter fixes them.

Yes, I can explain the motivation in the next version.