Re: [PATCH] acquire mmap semaphore in pagemap_read.

Martin Schwidefsky
2009-03-12T11:59:19Z

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.