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

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Date:   Thu, 22 Mar 2018 09:13:16 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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, 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.