Re: [PATCH] acquire mmap semaphore in pagemap_read.

Martin Schwidefsky
2009-03-12T15:32:20Z

On Thu, 12 Mar 2009 10:23:34 -0500
Matt Mackall <mpm@selenic.com> wrote:

> On Thu, 2009-03-12 at 12:54 +0100, Martin Schwidefsky wrote:
> > On Thu, 12 Mar 2009 14:45:33 +0300
> > Alexey Dobriyan <adobriyan@gmail.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.
>
> Well it means we may have to reintroduce the very annoying double
> buffering from various earlier implementations. But let's leave this
> discussion until after we've figured out what to do about the walker
> code.

Which would be really ugly. I still have not grasped why this will
introduce a deadlock though. The worst the put_user can do is to cause
a page fault, no? I do not see where the fault handler acquires the
mmap_sem as writer. It takes the mmap_sem as reader and two readers
should be fine.