[PATCH v2] fix __set_page_dirty_no_writeback() return value

[PATCH v2] fix __set_page_dirty_no_writeback() return value

[Bob Liu]

__set_page_dirty_no_writeback() should return true if it actually transitioned
the page from a clean to dirty state although it seems nobody used its return
value now.

Change from v1:
	* preserving cacheline optimisation as Andrew pointed out

Signed-off-by: Bob Liu <lliubbo@gmail.com>
---
 mm/page-writeback.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index bf85062..ac7018a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
  */
 int __set_page_dirty_no_writeback(struct page *page)
 {
-	if (!PageDirty(page))
+	if (!PageDirty(page)) {
 		SetPageDirty(page);
+		return 1;
+	}
 	return 0;
 }
 
-- 
1.6.3.3

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Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Andrew Morton]

On Thu, 11 Nov 2010 11:05:54 +0800 Bob Liu <lliubbo@gmail.com> wrote:

> __set_page_dirty_no_writeback() should return true if it actually transitioned
> the page from a clean to dirty state although it seems nobody used its return
> value now.
> 
> Change from v1:
> 	* preserving cacheline optimisation as Andrew pointed out
> 
> Signed-off-by: Bob Liu <lliubbo@gmail.com>
> ---
>  mm/page-writeback.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index bf85062..ac7018a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>   */
>  int __set_page_dirty_no_writeback(struct page *page)
>  {
> -	if (!PageDirty(page))
> +	if (!PageDirty(page)) {
>  		SetPageDirty(page);
> +		return 1;
> +	}
>  	return 0;
>  }

But that has a race.  If someone else sets PG_Dirty between the test
and the set, this function will incorrectly return 1.

Which is why it should use test_and_set if we're going to do this.

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Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Bob Liu]

On Thu, Nov 11, 2010 at 11:02 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 11 Nov 2010 11:05:54 +0800 Bob Liu <lliubbo@gmail.com> wrote:
>
>> __set_page_dirty_no_writeback() should return true if it actually transitioned
>> the page from a clean to dirty state although it seems nobody used its return
>> value now.
>>
>> Change from v1:
>>       * preserving cacheline optimisation as Andrew pointed out
>>
>> Signed-off-by: Bob Liu <lliubbo@gmail.com>
>> ---
>>  mm/page-writeback.c |    4 +++-
>>  1 files changed, 3 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index bf85062..ac7018a 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>>   */
>>  int __set_page_dirty_no_writeback(struct page *page)
>>  {
>> -     if (!PageDirty(page))
>> +     if (!PageDirty(page)) {
>>               SetPageDirty(page);
>> +             return 1;
>> +     }
>>       return 0;
>>  }
>
> But that has a race.  If someone else sets PG_Dirty between the test
> and the set, this function will incorrectly return 1.
>
> Which is why it should use test_and_set if we're going to do this.
>

Oh, Sorry for that.
I will make a new patch soon.

-- 
Thanks,
--Bob

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Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Wu Fengguang]

On Thu, Nov 11, 2010 at 11:05:54AM +0800, Bob Liu wrote:
> __set_page_dirty_no_writeback() should return true if it actually transitioned
> the page from a clean to dirty state although it seems nobody used its return
> value now.
> 
> Change from v1:
> 	* preserving cacheline optimisation as Andrew pointed out
> 
> Signed-off-by: Bob Liu <lliubbo@gmail.com>
> ---
>  mm/page-writeback.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index bf85062..ac7018a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>   */
>  int __set_page_dirty_no_writeback(struct page *page)
>  {
> -	if (!PageDirty(page))
> +	if (!PageDirty(page)) {
>  		SetPageDirty(page);
> +		return 1;
> +	}
>  	return 0;
>  }

It's still racy if not using TestSetPageDirty(). In fact
set_page_dirty() has a default reference implementation:

        if (!PageDirty(page)) {
                if (!TestSetPageDirty(page))
                        return 1;
        }
        return 0;

It seems the return value currently is only tested for doing
balance_dirty_pages_ratelimited(). So not a big problem.

Thanks,
Fengguang

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Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Bob Liu]

On Thu, Nov 11, 2010 at 11:26 AM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> On Thu, Nov 11, 2010 at 11:05:54AM +0800, Bob Liu wrote:
>> __set_page_dirty_no_writeback() should return true if it actually transitioned
>> the page from a clean to dirty state although it seems nobody used its return
>> value now.
>>
>> Change from v1:
>>       * preserving cacheline optimisation as Andrew pointed out
>>
>> Signed-off-by: Bob Liu <lliubbo@gmail.com>
>> ---
>>  mm/page-writeback.c |    4 +++-
>>  1 files changed, 3 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index bf85062..ac7018a 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>>   */
>>  int __set_page_dirty_no_writeback(struct page *page)
>>  {
>> -     if (!PageDirty(page))
>> +     if (!PageDirty(page)) {
>>               SetPageDirty(page);
>> +             return 1;
>> +     }
>>       return 0;
>>  }
>
> It's still racy if not using TestSetPageDirty(). In fact
> set_page_dirty() has a default reference implementation:

Yes, Andrew had also pointed out that. And I have send v3 fix this.
Could you ack it?

>
>        if (!PageDirty(page)) {
>                if (!TestSetPageDirty(page))
>                        return 1;

return !TestSetPageDirty(page) is more simply?

>        }
>        return 0;
>
> It seems the return value currently is only tested for doing
> balance_dirty_pages_ratelimited(). So not a big problem.
>

yeah, all those are small changes no matter with any problem:-).

-- 
Thanks,
--Bob

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Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Wu Fengguang]

On Thu, Nov 11, 2010 at 11:36:48AM +0800, Bob Liu wrote:
> On Thu, Nov 11, 2010 at 11:26 AM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> > On Thu, Nov 11, 2010 at 11:05:54AM +0800, Bob Liu wrote:
> >> __set_page_dirty_no_writeback() should return true if it actually transitioned
> >> the page from a clean to dirty state although it seems nobody used its return
> >> value now.
> >>
> >> Change from v1:
> >> A  A  A  * preserving cacheline optimisation as Andrew pointed out
> >>
> >> Signed-off-by: Bob Liu <lliubbo@gmail.com>
> >> ---
> >> A mm/page-writeback.c | A  A 4 +++-
> >> A 1 files changed, 3 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >> index bf85062..ac7018a 100644
> >> --- a/mm/page-writeback.c
> >> +++ b/mm/page-writeback.c
> >> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
> >> A  */
> >> A int __set_page_dirty_no_writeback(struct page *page)
> >> A {
> >> - A  A  if (!PageDirty(page))
> >> + A  A  if (!PageDirty(page)) {
> >> A  A  A  A  A  A  A  SetPageDirty(page);
> >> + A  A  A  A  A  A  return 1;
> >> + A  A  }
> >> A  A  A  return 0;
> >> A }
> >
> > It's still racy if not using TestSetPageDirty(). In fact
> > set_page_dirty() has a default reference implementation:
> 
> Yes, Andrew had also pointed out that. And I have send v3 fix this.
> Could you ack it?

Acked-by: Wu Fengguang <fengguang.wu@intel.com>

Thanks!

> >
> > A  A  A  A if (!PageDirty(page)) {
> > A  A  A  A  A  A  A  A if (!TestSetPageDirty(page))
> > A  A  A  A  A  A  A  A  A  A  A  A return 1;
> 
> return !TestSetPageDirty(page) is more simply?

Yeah that's fine.

> > A  A  A  A }
> > A  A  A  A return 0;
> >
> > It seems the return value currently is only tested for doing
> > balance_dirty_pages_ratelimited(). So not a big problem.
> >
> 
> yeah, all those are small changes no matter with any problem:-).

It's always good to make it correct :) I looked at the users mainly to
answer the question: is it a must fix for 2.6.37 or even 2.6.36.x?

Thanks,
Fengguang

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Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

[Bob Liu]

On Thu, Nov 11, 2010 at 11:45 AM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> On Thu, Nov 11, 2010 at 11:36:48AM +0800, Bob Liu wrote:
>> On Thu, Nov 11, 2010 at 11:26 AM, Wu Fengguang <fengguang.wu@intel.com> wrote:
>> > On Thu, Nov 11, 2010 at 11:05:54AM +0800, Bob Liu wrote:
>> >> __set_page_dirty_no_writeback() should return true if it actually transitioned
>> >> the page from a clean to dirty state although it seems nobody used its return
>> >> value now.
>> >>
>> >> Change from v1:
>> >>       * preserving cacheline optimisation as Andrew pointed out
>> >>
>> >> Signed-off-by: Bob Liu <lliubbo@gmail.com>
>> >> ---
>> >>  mm/page-writeback.c |    4 +++-
>> >>  1 files changed, 3 insertions(+), 1 deletions(-)
>> >>
>> >> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> >> index bf85062..ac7018a 100644
>> >> --- a/mm/page-writeback.c
>> >> +++ b/mm/page-writeback.c
>> >> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>> >>   */
>> >>  int __set_page_dirty_no_writeback(struct page *page)
>> >>  {
>> >> -     if (!PageDirty(page))
>> >> +     if (!PageDirty(page)) {
>> >>               SetPageDirty(page);
>> >> +             return 1;
>> >> +     }
>> >>       return 0;
>> >>  }
>> >
>> > It's still racy if not using TestSetPageDirty(). In fact
>> > set_page_dirty() has a default reference implementation:
>>
>> Yes, Andrew had also pointed out that. And I have send v3 fix this.
>> Could you ack it?
>
> Acked-by: Wu Fengguang <fengguang.wu@intel.com>
>

Thanks.

> Thanks!
>
>> >
>> >        if (!PageDirty(page)) {
>> >                if (!TestSetPageDirty(page))
>> >                        return 1;
>>
>> return !TestSetPageDirty(page) is more simply?
>
> Yeah that's fine.
>
>> >        }
>> >        return 0;
>> >
>> > It seems the return value currently is only tested for doing
>> > balance_dirty_pages_ratelimited(). So not a big problem.
>> >
>>
>> yeah, all those are small changes no matter with any problem:-).
>
> It's always good to make it correct :) I looked at the users mainly to
> answer the question: is it a must fix for 2.6.37 or even 2.6.36.x?
>

I have no idea, I think either is okay since it's not related with any bug.

-- 
Thanks,
--Bob

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