linux-kernel - Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

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Message-ID: <20180322161246.GG23100@dhcp22.suse.cz>
Date:   Thu, 22 Mar 2018 17:12:46 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

On Thu 22-03-18 09:06:14, Yang Shi wrote:
> 
> 
> On 3/22/18 2:10 AM, Michal Hocko wrote:
> > On Wed 21-03-18 15:36:12, Yang Shi wrote:
> > > 
> > > On 3/21/18 2:23 PM, Michal Hocko wrote:
[...]
> > > > pages and that is quite easy to move out of the write lock. That would
> > > > be an improvement already and it should be risk safe. If even that is
> > > > not sufficient then using range locking should help a lot. There
> > > > shouldn't be really any other address space operations within the range
> > > > most of the time so this would be basically non-contended access.
> > > It might depend on how the range is defined. Too big range may lead to
> > > surprisingly more contention, but too small range may bring in too much
> > > lock/unlock operations.
> > The full vma will have to be range locked. So there is nothing small or large.
> 
> It sounds not helpful to a single large vma case since just one range lock
> for the vma, it sounds equal to mmap_sem.

This is not how the range locking works. If we have a range lock per mm
then exclusive ranges are not contending. So if you are unmapping one
vma and want to create a new mapping or fault into a different range
then you are basically lockless.

-- 
Michal Hocko
SUSE Labs