linux-kernel - Re: [PATCH] acquire mmap semaphore in pagemap

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Message-ID: <20090312125410.25400d18@skybase>
Date:	Thu, 12 Mar 2009 12:54:10 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Matt Mackall <mpm@...enic.com>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] acquire mmap semaphore in pagemap_read.

On Thu, 12 Mar 2009 14:45:33 +0300
Alexey Dobriyan <adobriyan@...il.com> wrote:

> On Thu, Mar 12, 2009 at 11:33:08AM +0100, Martin Schwidefsky wrote:
> > --- linux-2.6/fs/proc/task_mmu.c
> > +++ linux-2.6-patched/fs/proc/task_mmu.c
> > @@ -716,7 +716,9 @@ static ssize_t pagemap_read(struct file 
> >  	 * user buffer is tracked in "pm", and the walk
> >  	 * will stop when we hit the end of the buffer.
> >  	 */
> > +	down_read(&mm->mmap_sem);
> >  	ret = walk_page_range(start_vaddr, end_vaddr, &pagemap_walk);
> > +	up_read(&mm->mmap_sem);
> 
> This will introduce "put_user under mmap_sem" which is deadlockable.

Hmm, interesting. In this case the pagemap interface is fundamentally broken.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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