[PATCH] mm: Add error check after call to rmap_walk in the function page_referenced

[PATCH] mm: Add error check after call to rmap_walk in the function page_referenced

[Nicholas Krause]

This adds a return check after the call to the function rmap_walk
in the function page_referenced as this function call can fail
and thus should signal callers of page_referenced if this happens
by returning the SWAP macro return value as returned by rmap_walk
here. In addition also check if have locked the page pointer as
passed to this particular and unlock it with unlock_page if this
page is locked before returning our SWAP marco return code from
rmap_walk.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
---
 mm/rmap.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 171b687..e4df848 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -814,7 +814,9 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
  * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
  *
  * Quick test_and_clear_referenced for all mappings to a page,
- * returns the number of ptes which referenced the page.
+ * returns the number of ptes which referenced the page.On
+ * error returns either zero or the error code returned from
+ * the failed call to rmap_walk.
  */
 int page_referenced(struct page *page,
 		    int is_locked,
@@ -855,7 +857,13 @@ int page_referenced(struct page *page,
 		rwc.invalid_vma = invalid_page_referenced_vma;
 	}
 
+
 	ret = rmap_walk(page, &rwc);
+	if (!ret) {
+		if (we_locked)
+			unlock_page(page);
+		return ret;
+	}
 	*vm_flags = pra.vm_flags;
 
 	if (we_locked)
-- 
2.1.4

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Re: [PATCH] mm: Add error check after call to rmap_walk in the function page_referenced

[Dominik Dingel]

On Thu, 25 Jun 2015 21:36:37 -0400
Nicholas Krause <xerofoify@gmail.com> wrote:

> This adds a return check after the call to the function rmap_walk
> in the function page_referenced as this function call can fail
> and thus should signal callers of page_referenced if this happens
> by returning the SWAP macro return value as returned by rmap_walk
> here. In addition also check if have locked the page pointer as
> passed to this particular and unlock it with unlock_page if this
> page is locked before returning our SWAP marco return code from
> rmap_walk.
> 
> Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
> ---
>  mm/rmap.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 171b687..e4df848 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -814,7 +814,9 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
>   * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
>   *
>   * Quick test_and_clear_referenced for all mappings to a page,
> - * returns the number of ptes which referenced the page.
> + * returns the number of ptes which referenced the page.On
> + * error returns either zero or the error code returned from
> + * the failed call to rmap_walk.
>   */
>  int page_referenced(struct page *page,
>  		    int is_locked,
> @@ -855,7 +857,13 @@ int page_referenced(struct page *page,
>  		rwc.invalid_vma = invalid_page_referenced_vma;
>  	}
> 
> +

unnecessary empty line

>  	ret = rmap_walk(page, &rwc);
> +	if (!ret) {
> +		if (we_locked)
> +			unlock_page(page);
> +		return ret;
> +	}

I don't see why the function should propagate the rmap_walk return value.
rmap_walk will not set pra.referenced, so that both callers just skip.

What is the purpose of the given patch? Do you have any real case introducing such code,
which is imho incomplete as all callers need to take care of the changed return value!

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Re: [PATCH] mm: Add error check after call to rmap_walk in the function page_referenced

[nick]

On 2015-06-26 09:56 AM, Dominik Dingel wrote:
> On Thu, 25 Jun 2015 21:36:37 -0400
> Nicholas Krause <xerofoify@gmail.com> wrote:
> 
>> This adds a return check after the call to the function rmap_walk
>> in the function page_referenced as this function call can fail
>> and thus should signal callers of page_referenced if this happens
>> by returning the SWAP macro return value as returned by rmap_walk
>> here. In addition also check if have locked the page pointer as
>> passed to this particular and unlock it with unlock_page if this
>> page is locked before returning our SWAP marco return code from
>> rmap_walk.
>>
>> Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
>> ---
>>  mm/rmap.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 171b687..e4df848 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -814,7 +814,9 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
>>   * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
>>   *
>>   * Quick test_and_clear_referenced for all mappings to a page,
>> - * returns the number of ptes which referenced the page.
>> + * returns the number of ptes which referenced the page.On
>> + * error returns either zero or the error code returned from
>> + * the failed call to rmap_walk.
>>   */
>>  int page_referenced(struct page *page,
>>  		    int is_locked,
>> @@ -855,7 +857,13 @@ int page_referenced(struct page *page,
>>  		rwc.invalid_vma = invalid_page_referenced_vma;
>>  	}
>>
>> +
> 
> unnecessary empty line
> 
>>  	ret = rmap_walk(page, &rwc);
>> +	if (!ret) {
>> +		if (we_locked)
>> +			unlock_page(page);
>> +		return ret;
>> +	}
> 
> I don't see why the function should propagate the rmap_walk return value.
> rmap_walk will not set pra.referenced, so that both callers just skip.
> 
> What is the purpose of the given patch? Do you have any real case introducing such code,
> which is imho incomplete as all callers need to take care of the changed return value!
> 
There is only one caller that needs to be moved over if this case is put in. Further more 
do we care if executing rmap_walk fails as if it does this means we were unable to execute
the function page_referenced one on the rmap_walk_control structure rwc and this can be
a issue in my option, if not then we can just remove the ret variable and execute rmap_walk
without checking it's return value.
Cheers Nick 

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Re: [PATCH] mm: Add error check after call to rmap_walk in the function page_referenced

[Dominik Dingel]

On Fri, 26 Jun 2015 10:47:39 -0400
nick <xerofoify@gmail.com> wrote:

> 
> 
> On 2015-06-26 09:56 AM, Dominik Dingel wrote:
> > On Thu, 25 Jun 2015 21:36:37 -0400
> > Nicholas Krause <xerofoify@gmail.com> wrote:
> > 
> >> This adds a return check after the call to the function rmap_walk
> >> in the function page_referenced as this function call can fail
> >> and thus should signal callers of page_referenced if this happens
> >> by returning the SWAP macro return value as returned by rmap_walk
> >> here. In addition also check if have locked the page pointer as
> >> passed to this particular and unlock it with unlock_page if this
> >> page is locked before returning our SWAP marco return code from
> >> rmap_walk.
> >>
> >> Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
> >> ---
> >>  mm/rmap.c | 10 +++++++++-
> >>  1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index 171b687..e4df848 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -814,7 +814,9 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
> >>   * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
> >>   *
> >>   * Quick test_and_clear_referenced for all mappings to a page,
> >> - * returns the number of ptes which referenced the page.
> >> + * returns the number of ptes which referenced the page.On
> >> + * error returns either zero or the error code returned from
> >> + * the failed call to rmap_walk.
> >>   */
> >>  int page_referenced(struct page *page,
> >>  		    int is_locked,
> >> @@ -855,7 +857,13 @@ int page_referenced(struct page *page,
> >>  		rwc.invalid_vma = invalid_page_referenced_vma;
> >>  	}
> >>
> >> +
> > 
> > unnecessary empty line
> > 
> >>  	ret = rmap_walk(page, &rwc);
> >> +	if (!ret) {
> >> +		if (we_locked)
> >> +			unlock_page(page);
> >> +		return ret;
> >> +	}
> > 
> > I don't see why the function should propagate the rmap_walk return value.
> > rmap_walk will not set pra.referenced, so that both callers just skip.
> > 
> > What is the purpose of the given patch? Do you have any real case introducing such code,
> > which is imho incomplete as all callers need to take care of the changed return value!
> > 
> There is only one caller that needs to be moved over if this case is put in. Further more 
> do we care if executing rmap_walk fails as if it does this means we were unable to execute
> the function page_referenced one on the rmap_walk_control structure rwc and this can be
> a issue in my option, if not then we can just remove the ret variable and execute rmap_walk
> without checking it's return value.
> Cheers Nick 

Let me rephrase: what will happen after you return ret?
What will: 
- shrink_active_list 
- page_check_references 
now do?

For your second thinking it would be good to check how and why ret was introduced?

git log -L '/int page_referenced(/',/^}/:mm/rmap.c

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Re: [PATCH] mm: Add error check after call to rmap_walk in the function page_referenced

[nick]

On 2015-06-26 11:18 AM, Dominik Dingel wrote:
> On Fri, 26 Jun 2015 10:47:39 -0400
> nick <xerofoify@gmail.com> wrote:
> 
>>
>>
>> On 2015-06-26 09:56 AM, Dominik Dingel wrote:
>>> On Thu, 25 Jun 2015 21:36:37 -0400
>>> Nicholas Krause <xerofoify@gmail.com> wrote:
>>>
>>>> This adds a return check after the call to the function rmap_walk
>>>> in the function page_referenced as this function call can fail
>>>> and thus should signal callers of page_referenced if this happens
>>>> by returning the SWAP macro return value as returned by rmap_walk
>>>> here. In addition also check if have locked the page pointer as
>>>> passed to this particular and unlock it with unlock_page if this
>>>> page is locked before returning our SWAP marco return code from
>>>> rmap_walk.
>>>>
>>>> Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
>>>> ---
>>>>  mm/rmap.c | 10 +++++++++-
>>>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>> index 171b687..e4df848 100644
>>>> --- a/mm/rmap.c
>>>> +++ b/mm/rmap.c
>>>> @@ -814,7 +814,9 @@ static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
>>>>   * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
>>>>   *
>>>>   * Quick test_and_clear_referenced for all mappings to a page,
>>>> - * returns the number of ptes which referenced the page.
>>>> + * returns the number of ptes which referenced the page.On
>>>> + * error returns either zero or the error code returned from
>>>> + * the failed call to rmap_walk.
>>>>   */
>>>>  int page_referenced(struct page *page,
>>>>  		    int is_locked,
>>>> @@ -855,7 +857,13 @@ int page_referenced(struct page *page,
>>>>  		rwc.invalid_vma = invalid_page_referenced_vma;
>>>>  	}
>>>>
>>>> +
>>>
>>> unnecessary empty line
>>>
>>>>  	ret = rmap_walk(page, &rwc);
>>>> +	if (!ret) {
>>>> +		if (we_locked)
>>>> +			unlock_page(page);
>>>> +		return ret;
>>>> +	}
>>>
>>> I don't see why the function should propagate the rmap_walk return value.
>>> rmap_walk will not set pra.referenced, so that both callers just skip.
>>>
>>> What is the purpose of the given patch? Do you have any real case introducing such code,
>>> which is imho incomplete as all callers need to take care of the changed return value!
>>>
>> There is only one caller that needs to be moved over if this case is put in. Further more 
>> do we care if executing rmap_walk fails as if it does this means we were unable to execute
>> the function page_referenced one on the rmap_walk_control structure rwc and this can be
>> a issue in my option, if not then we can just remove the ret variable and execute rmap_walk
>> without checking it's return value.
>> Cheers Nick 
> 
> Let me rephrase: what will happen after you return ret?
> What will: 
> - shrink_active_list 
> - page_check_references 
> now do?
> 
> For your second thinking it would be good to check how and why ret was introduced?
> 
> git log -L '/int page_referenced(/',/^}/:mm/rmap.c
> 
After looking into this further we do need to return the reference count of the pages to the functions 
you pointed out. After looking at the commit that uses ret there seems to be no point in checking rmap_walk's
return value and therefore ret can just be removed. Please let me known if I am missing something. 
Nick

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