[BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Artem Bityutskiy]

Hi,

commit 05fe478dd04e02fa230c305ab9b5616669821dd3
Author: Nick Piggin <npiggin@suse.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@nokia.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;
        }

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.

Also, I think wbc->nr_to_write should be changed in all cases, not only
when wbc->sync_mode == WB_SYNC_NONE.

Also, I add a comment explaining why we do not stop writing back.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 mm/page-writeback.c |   21 +++++++++++++++------
 1 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b493db7..13a2b8e 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1051,13 +1051,22 @@ continue_unlock:
 				}
  			}
 
-			if (wbc->sync_mode == WB_SYNC_NONE) {
-				wbc->nr_to_write--;
-				if (wbc->nr_to_write <= 0) {
-					done = 1;
-					break;
-				}
+			if (nr_to_write > 0)
+				nr_to_write--;
+			else if (wbc->sync_mode == WB_SYNC_NONE) {
+				/*
+				 * We stop writing back only if we are not
+				 * doing integrity sync. In case of integrity
+				 * sync we have to keep going because someone
+				 * may be concurrently dirtying pages, and we
+				 * might have synced a lot of newly appeared
+				 * dirty pages, bud have not synced all of the
+				 * old dirty pages.
+				 */
+				done = 1;
+				break;
 			}
+
 			if (wbc->nonblocking && bdi_write_congested(bdi)) {
 				wbc->encountered_congestion = 1;
 				done = 1;
-- 
1.6.0.6

-- 
Best regards,
Artem Bityutskiy (Битюцкий Артём)

Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Nick Piggin]

On Tuesday 03 February 2009 19:42:22 Artem Bityutskiy wrote:
> Hi,
>
> commit 05fe478dd04e02fa230c305ab9b5616669821dd3
> Author: Nick Piggin <npiggin@suse.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@nokia.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.

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...

> 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?


> 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.

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.

Can you cc stable@kernel.org when a final version gets merged upstream
please?

>
> Also, I add a comment explaining why we do not stop writing back.
>
> Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
> ---
>  mm/page-writeback.c |   21 +++++++++++++++------
>  1 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index b493db7..13a2b8e 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1051,13 +1051,22 @@ continue_unlock:
>  				}
>   			}
>
> -			if (wbc->sync_mode == WB_SYNC_NONE) {
> -				wbc->nr_to_write--;
> -				if (wbc->nr_to_write <= 0) {
> -					done = 1;
> -					break;
> -				}
> +			if (nr_to_write > 0)
> +				nr_to_write--;
> +			else if (wbc->sync_mode == WB_SYNC_NONE) {
> +				/*
> +				 * We stop writing back only if we are not
> +				 * doing integrity sync. In case of integrity
> +				 * sync we have to keep going because someone
> +				 * may be concurrently dirtying pages, and we
> +				 * might have synced a lot of newly appeared
> +				 * dirty pages, bud have not synced all of the
> +				 * old dirty pages.
> +				 */
> +				done = 1;
> +				break;
>  			}
> +
>  			if (wbc->nonblocking && bdi_write_congested(bdi)) {
>  				wbc->encountered_congestion = 1;
>  				done = 1;
> --
> 1.6.0.6

Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Nick Piggin]

On Tuesday 03 February 2009 20:13:56 Nick Piggin wrote:
> On Tuesday 03 February 2009 19:42:22 Artem Bityutskiy wrote:
> > Hi,
> >
> > commit 05fe478dd04e02fa230c305ab9b5616669821dd3
> > Author: Nick Piggin <npiggin@suse.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@nokia.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.
>
> 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...
>
> > 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?
>
> > 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.
>
> 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.
>
> Can you cc stable@kernel.org when a final version gets merged upstream
> please?

What I mean to say is that while I think the patch is not a problem,
and actually should be merged if it is required for ubifs, I would
like to just see the changelog clarified...

Also if you add a nice comment there, maybe mention that nr_to_write
must reflect the number of pages written even for the WB_SYNC_ALL case
(arguably the data integrity issue which you comment on is more
obvious in comparison to the nr_to_write requirement of callers).

Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Artem Bityutskiy]

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@suse.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@nokia.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@kernel.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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Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Nick Piggin]

On Tuesday 03 February 2009 20:32:49 Artem Bityutskiy wrote:
> 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@suse.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@nokia.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.

No, you are probably right to blame me ;) but I just am not quite sure
if I read your description correctly.


> 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

Ah, ok drat yes I missed this. Indeed this is a stupid bug :(


> 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.

2 was more or less intentional but I glossed over ubifs. At any rate
I'm happy if you restore the old behaviour.


> 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.

OK, it just wasn't clear to me from the changelog (I missed noticing bug #1).


> > > 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@kernel.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.

Yes please put an Acked-by: Nick Piggin <npiggin@suse.de> on there.
But I found your comments in this mail to be easier to understand.
Perhaps incorporate them into the changelog?

Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Artem Bityutskiy]

On Tue, 2009-02-03 at 20:45 +1100, Nick Piggin wrote:
> > > Can you cc stable@kernel.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.
> 
> Yes please put an Acked-by: Nick Piggin <npiggin@suse.de> on there.
> But I found your comments in this mail to be easier to understand.
> Perhaps incorporate them into the changelog?

Will do and re-send it.

-- 
Best regards,
Artem Bityutskiy (Битюцкий Артём)

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