linux-kernel - Re: ext4-mm-delalloc-ext4-lock-reverse-fix

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Date:	Mon, 19 May 2008 10:54:50 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, adobriyan@...il.com,
	Dave Young <hidave.darkstar@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: ext4-mm-delalloc-ext4-lock-reverse-fix

On Mon, 2008-05-19 at 17:24 +0200, Jiri Slaby wrote:
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Mon, 19 May 2008 15:15:09 +0800 "Dave Young" <hidave.darkstar@...il.com> wrote:
> > 
> > > Hi,
> > > 
> > > With 2.6.26-rc2-mm1 I need to sync frequently. It's not normal as my
> > > understanding.
> > > 
> > > 1. scp a big file from a lan server. Firstly the speed is about 8M/s,
> > > but the speed will slow down to 100K/s at last. After I exec "sync",
> > > the speed will restore.
> > > 
> > > Is this a known issue?
> > > 
> > 
> > Yes, probably related to the "mkfs takes forever" problem.  I haven't
> > looked into it yet.  Nor has anyone else afaik.
> 
> Absolutely not sure what this breaks in ext4, but fixes the problem for me:
> ---
>  mm/page-writeback.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 040bc7c..789b6ad 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -881,7 +881,7 @@ int write_cache_pages(struct address_space *mapping,
>  	pagevec_init(&pvec, 0);
>  	if (wbc->range_cyclic) {
>  		index = mapping->writeback_index; /* Start from prev offset */
> -		end = wbc->range_end >> PAGE_CACHE_SHIFT;
> +		end = -1;
>  	} else {
>  		index = wbc->range_start >> PAGE_CACHE_SHIFT;
>  		end = wbc->range_end >> PAGE_CACHE_SHIFT;

Thanks, the patch does fixed the regression .

In the delalloc-ext4-lock-reverse.patch. It wants the
write_cache_pages() to do incremental writeout within the reserved
journal credits in case of delalyed allocation(turns on cyclic mode and
remembers starts from writeback_index). If the writeout caller sets the
range_end, it should not flushing to the end of the file in the case of
range_cyclic mode. The patch above fix the regression but makes possible
that we write too much. 


I'd say drop the delalloc-ext4-lock-reverse.patch for now, will re-think
of the delayed allocation support for page-journal-lock reserve method.

Mingming







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