Re: [PATCH] ext4: calculate journal credits correctly

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

On Wed, Nov 12, 2008 at 04:03:42PM -0800, Mingming Cao wrote:
> 
> 在 2008-11-06四的 14:51 -0500，Theodore Ts'o写道：
> > This fixes a 2.6.27 regression which was introduced in commit a02908f1.
> > 
> > We weren't passing the chunk parameter down to the two subections,
> > ext4_indirect_trans_blocks() and ext4_ext_index_trans_blocks(), with
> > the result that massively overestimate the amount of credits needed by
> > ext4_da_writepages, especially in the non-extents case.  This causes
> > failures especially on /boot partitions, which tend to be small and
> > non-extent using since GRUB doesn't handle extents.
> > 
> > Thanks to Joseph Fannin for reporting this bug.
> > 
> Thanks for fixing this up!

This was sent to my gmail.com  id so missed it before.
> 
> However, looking at the code path, I wasn't sure if we got the delalloc
> credits right.  In ext4_da_writepages()->mpage_da_writepages(),  the
> credits is calculated based on the assumption that mpage_da_writepages()
> doing* one* single chunk of contigugous allocation? So only one single
> extent credit is reserved (here you see the "chunk" flag passed from the
> ext4_da_writepages)
> 
> __mpage_da_writepage() does do single chunk of block allocation at a
> time, but mpage_da_writepages()->write_cache_pages() will loop the page
> vectors and probably will calling writepage->__mpage_da_writepage->
> mpage_da_map_blocks() multiple times?  Am I  missing anything?
> 

mpage_da_writepages does block allocation for single chunk only.
ext4_da_writepages allocate credit needed for single chunk allocation.
write_cache_pages iterate through contiguous pages and build an in
memory extent. It then call get_blocks for 'x' blocks. After that
we map the blocks to the unmapped buffer_heads. Then we do
ext4_da_writepage which redirty pages which are not fully mapped. That
means we skip pages. We retry using mpage_data_writepages again.

-aneesh
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