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

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Date:	Fri, 27 Feb 2015 10:34:46 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	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
Subject: Re: [PATCH] mm: replace mmap_sem for mm->exe_file serialization

On Fri, 2015-02-27 at 18:36 +0100, Oleg Nesterov wrote:
> 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?

I don't see why this is a random lock nor how would we regret this
later. I regret having to do these kind of patches because people were
lazy and just relied on mmap_sem without thinking beyond their use case.
As mentioned I'm also planning on creating an own sort of
exe_file_struct, which would be an isolated entity (still in the mm
though), with its own locking and prctl bits, that would tidy mm_struct
a bit. RCU was something else I considered, but it doesn't suite well in
all paths and we would still need a spinlock when updating the file
anyway.

If you have a better suggestion please do tell.

> 
> > 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 ;)

Its much beyond that. As mentioned, for any significant changes to the
mmap_sem locking scheme, this sort of thing needs to be addressed first.

Thanks,
Davidlohr

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