Re: [PATCH 2/3] btrfs: fix readdir deadlock with pagefault

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Jul 24, 2017 at 11:26:49AM +0300, Nikolay Borisov wrote:
> 
> 
> On 21.07.2017 20:29, josef@toxicpanda.com wrote:
> > From: Josef Bacik <jbacik@fb.com>
> > 
> > Readdir does dir_emit while under the btree lock.  dir_emit can trigger
> > the page fault which means we can deadlock.  Fix this by allocating a
> > buffer on opening a directory and copying the readdir into this buffer
> > and doing dir_emit from outside of the tree lock.
> 
> So dir_emit essentially calls filldir which can fault on the user
> provided addresses. How could a fault there recurse back to the filesystem?
> 

Thread A
readdir  <holding tree lock>
  dir_emit
    <page fault>
      down_read(mmap_sem)

Thread B
mmap write
  down_write(mmap_sem)
    page_mkwrite
      wait_ordered_extents

Process C
finish_ordered_extent
  insert_reserved_file_extent
   try to lock leaf <hang>

Thanks,

Josef