Re: [PATCH 5/5 v6] ext4: fix partial page writes

[Ted Ts'o <tytso@mit.edu>]

On Wed, Aug 24, 2011 at 12:07:22PM -0700, Allison Henderson wrote:
> While running extended fsx tests to verify the preceeding patches,
> a similar bug was also found in the write operation
> 
> When ever a write operation begins or ends in a hole,
> or extends EOF, the partial page contained in the hole
> or beyond EOF needs to be zeroed out.
> 
> To correct this the new ext4_discard_partial_page_buffers_no_lock
> routine is used to zero out the partial page, but only for buffer
> heads that are already unmapped.
> 
> Signed-off-by: Allison Henderson <achender@linux.vnet.ibm.com>

I haven't had a chance to debug this yet, but FYI, with this patch
applied, xfstests #224 causes OOM's to result when running under KVM
with 1024megs.  I tried doubling the memory to 2048 megs, and it
passed in 4k standard ext4 mode, but then got hit with the OOM killer
on test #224 in ext3 emulation mode (nodealloc, and with extents not
enabled in the file system).  Without this patch, and the first four
patches in the v6 patch series applied, xfstests #224 passes in all of
the standard configurations (4k, ext3, 1k, data=journal, etc.)

I don't know why this is happening, but I'll try to dig into it in the
next day or two.  Any thoughts or fixes you might suggest would be
much appreciated.

						- Ted