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Message-Id: <1233653569.24809.69.camel@localhost.localdomain>
Date: Tue, 03 Feb 2009 11:32:49 +0200
From: Artem Bityutskiy <dedekind@...radead.org>
To: Nick Piggin <nickpiggin@...oo.com.au>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>, npiggin@...e.de,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter
On Tue, 2009-02-03 at 20:13 +1100, Nick Piggin wrote:
> On Tuesday 03 February 2009 19:42:22 Artem Bityutskiy wrote:
> > Hi,
> >
> > commit 05fe478dd04e02fa230c305ab9b5616669821dd3
> > Author: Nick Piggin <npiggin@...e.de>
> > Date: Tue Jan 6 14:39:08 2009 -0800
> >
> > mm: write_cache_pages integrity fix
> >
> > broke wbc->nr_to_write handling. Here is the fix.
> >
> > I'm not 100% sure I got things right, because I am far not expert in the
> > area. Please, review it. The patch fixes my UBIFS issues, which are
> > caused by the fact that wbc->nr_to_write is not updated.
> > ======================================================================
> >
> > From: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
> > Date: Mon, 2 Feb 2009 18:33:49 +0200
> > Subject: [PATCH] write-back: fix nr_to_write counter
> >
> > Commit 05fe478dd04e02fa230c305ab9b5616669821dd3 broke @wbc->nr_to_write.
> > 'write_cache_pages()' changes it in the loop, but restores the original
> > value from @nr_to_write at the end, because of this code:
> >
> > if (!wbc->no_nrwrite_index_update) {
> > if (wbc->range_cyclic || (range_whole && nr_to_write > 0))
> > mapping->writeback_index = done_index;
> > wbc->nr_to_write = nr_to_write;
> > }
>
> The commit you quote only moves nr_to_write to not take effect for
> WB_SYNC_ALL (ie. data integrity) writeout. And makes no other change
> to write_cache_pages.
Nick, I'm sorry if my e-mail looked like I'm blame you, I referred the
commit because git-bisect pointed to it and I though it for me :-)
And I apologize if I write stupid things.
Here is the commit 05fe478dd04e02fa230c305ab9b5616669821dd3:
diff --git a/mm/filemap.c b/mm/filemap.c
index f9d8818..9c5e623 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -210,7 +210,7 @@ int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
int ret;
struct writeback_control wbc = {
.sync_mode = sync_mode,
- .nr_to_write = mapping->nrpages * 2,
+ .nr_to_write = LONG_MAX,
.range_start = start,
.range_end = end,
};
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2e847cd..5edca67 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -963,8 +963,10 @@ retry:
}
}
- if (--nr_to_write <= 0)
- done = 1;
+ if (wbc->sync_mode == WB_SYNC_NONE) {
+ if (--wbc->nr_to_write <= 0)
+ done = 1;
+ }
if (wbc->nonblocking && bdi_write_congested(bdi)) {
wbc->encountered_congestion = 1;
done = 1;
It makes the following changes:
1. Decrement wbc->nr_to_write instead of nr_to_write
2. Decrement wbc->nr_to_write _only_ if wbc->sync_mode == WB_SYNC_NONE
3. If synced nr_to_write pages, stop only if if wbc->sync_mode ==
WB_SYNC_NONE, otherwise keep going.
The commit message talks only about item 3, at leaset as I understand
it. I do not quote the commit message because it is large. Thus, I
thought changes 1 and 2 were not intentional.
In my patch I try to
1. Undo changes 1 and 2
2. Add a comment explaining change 3 (it very useful to have comments in
_code_, not only in the commit)
> I thought your problem might have been that you were calling this
> with WB_SYNC_ALL and expecting it to heed nr_to_write, however...
Err, my problem is that wbc->nr_to_write is not updated.
> > Also, I think wbc->nr_to_write should be changed in all cases, not only
> > when wbc->sync_mode == WB_SYNC_NONE.
>
> ... you mention this here like it is an *additional* issue on top
> of your problem. So I fail to see how my commit could have caused
> this problem?
This is the change number 2 in your commit.
> > Well, in case of @wbc->no_nrwrite_index_update != 0, we do change
> > wbc->nr_to_write, while we should not. This patch fixes this behavior.
>
> And I don't know what you mean by this because the patch doesn't
> fix any problem there AFAIKS.
This is about change number 1. In the "for" loop you change
wbc->nr_to_write, instead of nr_to_write. But before the function
returns, it writes nr_to_write back to wbc->nr_to_write, so the
result is that the caller sees wbc->nr_to_write unchanged.
> Anyway, I did probably not pay enough attention to ubifs when making
> this change, and if it wants wbc->nr_to_write updated even for data
> integrity syncs, I don't see the harm in that. So I don't have any
> objection to your patch. Thanks.
It is just how things were _before_ your patch.
> Can you cc stable@...nel.org when a final version gets merged upstream
> please?
Sure, I just want to get blessing of one of MM/FS gurus. E.g. you
Acked-by would be enough.
Thanks.
--
Best regards,
Artem Bityutskiy (Битюцкий Артём)
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