linux-kernel - Re: [PATCH] mm: replace mmap_sem for mm->exe

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Message-ID: <20150227173650.GA18823@redhat.com>
Date:	Fri, 27 Feb 2015 18:36:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Davidlohr Bueso <dave.bueso@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dave@...olabs.net
Subject: Re: [PATCH] mm: replace mmap_sem for mm->exe_file serialization

On 02/26, Cyrill Gorcunov wrote:
>
> On Thu, Feb 26, 2015 at 11:36:57AM -0800, Davidlohr Bueso wrote:
> > We currently use the mmap_sem to serialize the mm exe_file.
> > This is atrocious and a clear example of the misuses this
> > lock has all over the place, making any significant changes
> > to the address space locking that much more complex and tedious.
> > This also has to do of how we used to check for the vma's vm_file
> > being VM_EXECUTABLE (much of which was replaced by 2dd8ad81e31).
> >
> > This patch, therefore, removes the mmap_sem dependency and
> > introduces a specific lock for the exe_file (rwlock_t, as it is
> > read mostly and protects a trivial critical region). As mentioned,
> > the motivation is to cleanup mmap_sem (as opposed to exe_file
> > performance).

Well, I didn't see the patch, can't really comment.

But I have to admit that this looks as atrocious and a clear example of
"lets add yet another random lock which we will regret about later" ;)

rwlock_t in mm_struct just to serialize access to exe_file?

> A nice side effect of this is that we avoid taking
> > the mmap_sem (shared) in fork paths for the exe_file handling
> > (note that readers block when the rwsem is taken exclusively by
> > another thread).

Yes, this is ugly. Can't we kill this dup_mm_exe_file() and copy change
dup_mmap() to also dup ->exe_file ?

> Hi Davidlohr, it would be interesting to know if the cleanup
> bring some performance benefit?

To me the main question is whether the patch makes this code simpler
or uglier ;)

Oleg.

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