[patch] mm, migrate: increment fail count on ENOMEM

[patch] mm, migrate: increment fail count on ENOMEM

[David Rientjes]

If page migration fails due to -ENOMEM, nr_failed should still be
incremented for proper statistics.

This was encountered recently when all page migration vmstats showed 0,
and inferred that migrate_pages() was never called, although in reality
the first page migration failed because compaction_alloc() failed to find
a migration target.

This patch increments nr_failed so the vmstat is properly accounted on
ENOMEM.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/migrate.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/migrate.c b/mm/migrate.c
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 
 			switch(rc) {
 			case -ENOMEM:
+				nr_failed++;
 				goto out;
 			case -EAGAIN:
 				retry++;

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Re: [patch] mm, migrate: increment fail count on ENOMEM

[Michal Hocko]

On Thu 19-05-16 15:11:23, David Rientjes wrote:
> If page migration fails due to -ENOMEM, nr_failed should still be
> incremented for proper statistics.
> 
> This was encountered recently when all page migration vmstats showed 0,
> and inferred that migrate_pages() was never called, although in reality
> the first page migration failed because compaction_alloc() failed to find
> a migration target.
> 
> This patch increments nr_failed so the vmstat is properly accounted on
> ENOMEM.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

One question though

> ---
>  mm/migrate.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  
>  			switch(rc) {
>  			case -ENOMEM:
> +				nr_failed++;
>  				goto out;
>  			case -EAGAIN:
>  				retry++;

Why don't we need also to count also retries?
---
diff --git a/mm/migrate.c b/mm/migrate.c
index 53ab6398e7a2..ef9c5211ae3c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 			}
 		}
 	}
+out:
 	nr_failed += retry;
 	rc = nr_failed;
-out:
 	if (nr_succeeded)
 		count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
 	if (nr_failed)
-- 
Michal Hocko
SUSE Labs

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Re: [patch] mm, migrate: increment fail count on ENOMEM

[Vlastimil Babka]

On 05/20/2016 03:06 PM, Michal Hocko wrote:
> On Thu 19-05-16 15:11:23, David Rientjes wrote:
>> If page migration fails due to -ENOMEM, nr_failed should still be
>> incremented for proper statistics.
>>
>> This was encountered recently when all page migration vmstats showed 0,
>> and inferred that migrate_pages() was never called, although in reality
>> the first page migration failed because compaction_alloc() failed to find
>> a migration target.
>>
>> This patch increments nr_failed so the vmstat is properly accounted on
>> ENOMEM.
>>
>> Signed-off-by: David Rientjes <rientjes@google.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> One question though
>
>> ---
>>   mm/migrate.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>
>>   			switch(rc) {
>>   			case -ENOMEM:
>> +				nr_failed++;
>>   				goto out;
>>   			case -EAGAIN:
>>   				retry++;
>
> Why don't we need also to count also retries?

We could, but not like you suggest.

> ---
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 53ab6398e7a2..ef9c5211ae3c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   			}
>   		}
>   	}
> +out:
>   	nr_failed += retry;
>   	rc = nr_failed;

This overwrites rc == -ENOMEM, which at least compaction needs to 
recognize. But we could duplicate "nr_failed += retry" in the case -ENOMEM.

> -out:
>   	if (nr_succeeded)
>   		count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
>   	if (nr_failed)
>

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Re: [patch] mm, migrate: increment fail count on ENOMEM

[Michal Hocko]

On Fri 20-05-16 15:19:12, Vlastimil Babka wrote:
> On 05/20/2016 03:06 PM, Michal Hocko wrote:
[...]
> > Why don't we need also to count also retries?
> 
> We could, but not like you suggest.
> 
> > ---
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 53ab6398e7a2..ef9c5211ae3c 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >   			}
> >   		}
> >   	}
> > +out:
> >   	nr_failed += retry;
> >   	rc = nr_failed;
> 
> This overwrites rc == -ENOMEM, which at least compaction needs to recognize.
> But we could duplicate "nr_failed += retry" in the case -ENOMEM.

Right you are. So we should do
---
diff --git a/mm/migrate.c b/mm/migrate.c
index 53ab6398e7a2..123fed94022b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 
 			switch(rc) {
 			case -ENOMEM:
+				nr_failed += retry + 1;
 				goto out;
 			case -EAGAIN:
 				retry++;
	

-- 
Michal Hocko
SUSE Labs

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Re: [patch] mm, migrate: increment fail count on ENOMEM

[Andrew Morton]

On Fri, 20 May 2016 15:31:21 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Fri 20-05-16 15:19:12, Vlastimil Babka wrote:
> > On 05/20/2016 03:06 PM, Michal Hocko wrote:
> [...]
> > > Why don't we need also to count also retries?
> > 
> > We could, but not like you suggest.
> > 
> > > ---
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 53ab6398e7a2..ef9c5211ae3c 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> > >   			}
> > >   		}
> > >   	}
> > > +out:
> > >   	nr_failed += retry;
> > >   	rc = nr_failed;
> > 
> > This overwrites rc == -ENOMEM, which at least compaction needs to recognize.
> > But we could duplicate "nr_failed += retry" in the case -ENOMEM.
> 
> Right you are. So we should do
> ---
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 53ab6398e7a2..123fed94022b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  
>  			switch(rc) {
>  			case -ENOMEM:
> +				nr_failed += retry + 1;
>  				goto out;
>  			case -EAGAIN:
>  				retry++;
> 	
> 

argh, this was lost.  Please resend as a real patch sometime?

Thanks.

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Re: [patch] mm, migrate: increment fail count on ENOMEM

[Hugh Dickins]

On Mon, 23 May 2016, Andrew Morton wrote:
> On Fri, 20 May 2016 15:31:21 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> > On Fri 20-05-16 15:19:12, Vlastimil Babka wrote:
> > > On 05/20/2016 03:06 PM, Michal Hocko wrote:
> > [...]
> > > > Why don't we need also to count also retries?
> > > 
> > > We could, but not like you suggest.
> > > 
> > > > ---
> > > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > > index 53ab6398e7a2..ef9c5211ae3c 100644
> > > > --- a/mm/migrate.c
> > > > +++ b/mm/migrate.c
> > > > @@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> > > >   			}
> > > >   		}
> > > >   	}
> > > > +out:
> > > >   	nr_failed += retry;
> > > >   	rc = nr_failed;
> > > 
> > > This overwrites rc == -ENOMEM, which at least compaction needs to recognize.
> > > But we could duplicate "nr_failed += retry" in the case -ENOMEM.
> > 
> > Right you are. So we should do
> > ---
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 53ab6398e7a2..123fed94022b 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >  
> >  			switch(rc) {
> >  			case -ENOMEM:
> > +				nr_failed += retry + 1;
> >  				goto out;
> >  			case -EAGAIN:
> >  				retry++;
> > 	
> > 
> 
> argh, this was lost.  Please resend as a real patch sometime?

It's not correct.  "retry" is reset to 0 each time around the
loop, and it's only a meaningful number to add on to nr_failed, in
the case when we've gone through the whole list: not in this "goto
out" case.  We could add another loop to count how many are left
when we hit -ENOMEM, and add that on to nr_failed; but I'm not
convinced that it's worth the bother.

Hugh

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Re: [patch] mm, migrate: increment fail count on ENOMEM

[Michal Hocko]

On Mon 23-05-16 16:32:56, Hugh Dickins wrote:
> On Mon, 23 May 2016, Andrew Morton wrote:
> > On Fri, 20 May 2016 15:31:21 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> > > On Fri 20-05-16 15:19:12, Vlastimil Babka wrote:
> > > > On 05/20/2016 03:06 PM, Michal Hocko wrote:
> > > [...]
> > > > > Why don't we need also to count also retries?
> > > > 
> > > > We could, but not like you suggest.
> > > > 
> > > > > ---
> > > > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > > > index 53ab6398e7a2..ef9c5211ae3c 100644
> > > > > --- a/mm/migrate.c
> > > > > +++ b/mm/migrate.c
> > > > > @@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> > > > >   			}
> > > > >   		}
> > > > >   	}
> > > > > +out:
> > > > >   	nr_failed += retry;
> > > > >   	rc = nr_failed;
> > > > 
> > > > This overwrites rc == -ENOMEM, which at least compaction needs to recognize.
> > > > But we could duplicate "nr_failed += retry" in the case -ENOMEM.
> > > 
> > > Right you are. So we should do
> > > ---
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 53ab6398e7a2..123fed94022b 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> > >  
> > >  			switch(rc) {
> > >  			case -ENOMEM:
> > > +				nr_failed += retry + 1;
> > >  				goto out;
> > >  			case -EAGAIN:
> > >  				retry++;
> > > 	
> > > 
> > 
> > argh, this was lost.  Please resend as a real patch sometime?
> 
> It's not correct.  "retry" is reset to 0 each time around the
> loop, and it's only a meaningful number to add on to nr_failed, in
> the case when we've gone through the whole list: not in this "goto
> out" case. 

You are right! I've missed that, my bad.

> We could add another loop to count how many are left
> when we hit -ENOMEM, and add that on to nr_failed; but I'm not
> convinced that it's worth the bother.

Agreed!

Sorry about the noise!
-- 
Michal Hocko
SUSE Labs

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