linux-kernel - Re: [RFC][PATCH] mm: write_cache

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Message-ID: <20090907144514.GC28054@wotan.suse.de>
Date:	Mon, 7 Sep 2009 16:45:14 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Adrian Hunter <adrian.hunter@...ia.com>
Cc:	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"david@...morbit.com" <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Bityutskiy Artem (Nokia-D/Helsinki)" <Artem.Bityutskiy@...ia.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] mm: write_cache_pages be more sequential

On Mon, Sep 07, 2009 at 05:29:07PM +0300, Adrian Hunter wrote:
> Nick Piggin wrote:
> >On Mon, Sep 07, 2009 at 05:07:38PM +0300, Adrian Hunter wrote:
> >>>From 6f3bb7c26936c45d810048f59c369e8d5a5623fc Mon Sep 17 00:00:00 2001
> >>From: Adrian Hunter <adrian.hunter@...ia.com>
> >>Date: Mon, 7 Sep 2009 10:49:11 +0300
> >>Subject: [PATCH] mm: write_cache_pages be more sequential
> >>
> >>If a file is written to sequentially, then writeback
> >>should write the pages sequentially also.  However,
> >>that does not always happen.  For example:
> >>
> >>1) user writes pages 0, 1 and 2 but 2 is incomplete
> >>2) write_cache_pages writes pages 0, 1 and 2 and sets
> >>writeback_index to 3
> >>3) user finishes writing page 2 and writes pages 3 and 4
> >>4) write_cache_pages writes pages 3 and 4, and then cycles
> >>back and writes page 2 again.
> >>
> >>So the pages are written out in the order 0, 1, 2, 3 ,4 ,2
> >>instead of 0, 1, 2, 2, 3, 4.
> >
> >Why does page 2 get set dirty if the write was incomplete?
> 
> I meant that only part of the page was written. e.g.
> write 10240 bytes, wait for writeback, then write another
> 10240 bytes.  The pages will be written out in the order
> 0, 1, 2, 3, 4, 2

OK...


> >>This situation was noticed on UBIFS because it writes
> >>directly from writepage.  Hence if there is an unexpected
> >>power-loss, a file will end up with a hole even though
> >>the file was written sequentially by the user.
> >>
> >>Signed-off-by: Adrian Hunter <adrian.hunter@...ia.com>
> >>---
> >>mm/page-writeback.c |    2 ++
> >>1 files changed, 2 insertions(+), 0 deletions(-)
> >>
> >>diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >>index 81627eb..7410b7a 100644
> >>--- a/mm/page-writeback.c
> >>+++ b/mm/page-writeback.c
> >>@@ -960,6 +960,8 @@ int write_cache_pages(struct address_space *mapping,
> >>	pagevec_init(&pvec, 0);
> >>	if (wbc->range_cyclic) {
> >>		writeback_index = mapping->writeback_index; /* prev offset */
> >>+		if (writeback_index)
> >>+			writeback_index -= 1;
> >>		index = writeback_index;
> >>		if (index == 0)
> >>			cycled = 1;
> >
> >Doesn't this just break range_cyclic? range_cyclic is supposed to
> >work across calls to write_cache_pages, and it's there I guess so
> >background writeout will be able to eventually get around to writing
> >all pages relatively fairly in the presence of redirtying operations.
> 
> I do not immediately see how it breaks range_cyclic.  Can you give an
> example?

Oh, I must be dyslexic, I read it as writeback_index = -1; :P
But I think it can still cause some subtle problems with error
cases.

I guess you could just make the done_index assignment more logical
and make it page->index. Then add a comment when assigning to
writeback_index that you want to start up again at the previously
written page to help this case.

Also, check to ensure the error cases are going to still work correctly.
Eg. you might want to increment done_index in the case of error.

I guess it is a reasonable workaround for the problem. It is a bit
unsatisfying to special case on a page basis like this, but anyway
I don't think there should be a realistic downside in practice.

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