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Re: [PATCH 3/4] writeback: pay attention to wbc->nr_to_write in write_ca

To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH 3/4] writeback: pay attention to wbc->nr_to_write in write_cache_pages
From: tytso@xxxxxxx
Date: Sat, 1 May 2010 15:47:10 -0400
Cc: "Aneesh Kumar K. V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20100430124329.10a4c02b.akpm@xxxxxxxxxxxxxxxxxxxx>
Mail-followup-to: tytso@xxxxxxx, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, "Aneesh Kumar K. V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
References: <1271731314-5893-1-git-send-email-david@xxxxxxxxxxxxx> <1271731314-5893-4-git-send-email-david@xxxxxxxxxxxxx> <20100429143931.331c2bab.akpm@xxxxxxxxxxxxxxxxxxxx> <87sk6dwka6.fsf@xxxxxxxxxxxxxxxxxx> <20100430124329.10a4c02b.akpm@xxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Fri, Apr 30, 2010 at 12:43:29PM -0700, Andrew Morton wrote:
> 
> Maybe that fs shouldn't be calling write_cache_pages() at all.  After
> all, write_cache_pages() is a wrapper which emits a sequence of calls
> to ->writepage(), and ->writepage() writes a page.

On my todo list is to fix ext4 to not call write_cache_pages() at all.
We are seriously abusing that function ATM, since we're not actually
writing the pages when we call write_cache_pages().  I won't go into
what we're doing, because it's too embarassing, but suffice it to say
that we end up calling pagevec_lookup() or pagevec_lookup_tag()
*four*, count them *four* times while trying to do writeback.

I have a simple patch that gives ext4 our own copy of
write_cache_pages(), and then simplifies it a lot, and fixes a bunch
of problems, but then I discarded it in favor of fundamentally redoing
how we do writeback at all, but it's going to take a while to get
things completely right.  But I am working to try to fix this.

If it would help, I can ressurect the "fork write_cache_pages() and
simplify" patch, so ext4 isn't dependent on the mm/page-writeback.c's
write_cache_pages(), if there is an immediate, short-term need to fix
that function.

                                                - Ted

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