Re: [PATCH] ext4: When reading from fallocated blocks make sure we return zero.

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Sat, Feb 16, 2008 at 08:53:34AM +0530, Aneesh Kumar K.V wrote:
> On Fri, Feb 15, 2008 at 11:43:04AM -0800, Mingming Cao wrote:
> > On Fri, 2008-02-15 at 23:46 +0530, Aneesh Kumar K.V wrote:
> > > fallocate blocks are considered as sparse area and read from them should
> > > return zero. ext4_ext_get_blocks should return zero for read request.
> > > 
> > 
> > The patch itself looks harmless, but I still don't see how this could
> > fix the problem you described at irc: a write hit a BUG_ON() in
> > fs/buffer.c saying the buffer is not mapped. Could you add more details
> > here?
> 
> Write will take the below call chain
> 
> ext4_write_begin
>   block_write_begin
>     __block_prepare_write
>        ext4_getblock
>          ext4_get_blocks_wrap
> (1)	   ext4_ext_get_blocks with create = 0 return allocated
>        ll_rw_block  if buffer not uptodate.
>          submit_bh
> 	   BUG_ON(!buffer_mapped(bh))
> 
> 
> ext4_ext_get_blocks at (1) should have returned 0. That would cause
> ext4_get_blocks_wrap to again call ext4_ext_get_blocks with create = 1
> and that would have returned us the buffer head which is mapped. This
> would also result in splitting the extent to initialized and
> uninitialized one.
> 

The change is also needed to get mmap on fallocate space to work.

------------[ cut here ]------------
WARNING: at fs/buffer.c:1680 __block_write_full_page+0x101/0x2f1()
 Modules linked in:
 Pid: 2478, comm: mmaptest Not tainted 2.6.25-rc1 #12
  [<c0120e84>] warn_on_slowpath+0x41/0x51
  [<c0108c00>] ?  native_sched_clock+0x2d/0x9f
  [<c013b44e>] ?  __lock_acquire+0xacb/0xb13
  [<c013b44e>] ?  __lock_acquire+0xacb/0xb13
  [<c0180f97>] __block_write_full_page+0x101/0x2f1
  [<c01d053f>] ?  ext4_get_block+0x0/0xc0
  [<c018124f>] block_write_full_page+0xc8/0xd1
  [<c01d053f>] ?  ext4_get_block+0x0/0xc0
  [<c01d1a36>] ext4_ordered_writepage+0xad/0x146
  [<c01cec12>] ? bget_one+0x0/0xb
  [<c014c5dd>] __writepage+0xb/0x25
  [<c014cab2>] write_cache_pages+0x180/0x287
  [<c014c5d2>] ? __writepage+0x0/0x25
  [<c01527d5>] ?  __do_fault+0x2e2/0x324
  [<c0147889>] ? unlock_page+0x25/0x28 
  [<c014cbd6>] generic_writepages+0x1d/0x27
  [<c014cc0c>] do_writepages+0x2c/0x34
  [<c0147fe1>] __filemap_fdatawrite_range+0x5b/0x67
  [<c01481ba>] filemap_fdatawrite+0x15/0x17
  [<c017ea3d>] do_fsync+0x2c/0x9a
  [<c017eacb>] __do_fsync+0x20/0x2f
  [<c017eaf9>] sys_fsync+0xd/0xf
  [<c0104992>] sysenter_past_esp+0x5f/0xa5
  =======================
 ---[ end trace 5ba60b430e0af601 ]---


-aneesh