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Date: Thu, 22 Mar 2018 09:13:16 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section
Message-ID: <20180322161316.GD28468@bombadil.infradead.org>
References: <1521581486-99134-1-git-send-email-yang.shi@linux.alibaba.com>
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 <cf87ade4-5a5c-3919-0fc6-acc40e12659b@linux.alibaba.com>
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To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Michal Hocko <mhocko@kernel.org>, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org

On Thu, Mar 22, 2018 at 09:06:14AM -0700, 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:
> > > > On Wed 21-03-18 10:16:41, Yang Shi wrote:
> > > > > proc_pid_cmdline_read(), it calls access_remote_vm() which need acquire
> > > > > mmap_sem too, so the mmap_sem scalability issue will be hit sooner or later.
> > > > Ohh, absolutely. mmap_sem is unfortunatelly abused and it would be great
> > > > to remove that. munmap should perform much better. How to do that safely
> > 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.

But splitting mmap_sem into pieces is beneficial for this case.  Imagine
we have a spinlock / rwlock to protect the rbtree / arg_start / arg_end
/ ...  and then each VMA has a rwsem (or equivalent).  access_remote_vm()
would walk the tree and grab the VMA's rwsem for read while reading
out the arguments.  The munmap code would have a completely different
VMA write-locked.