[PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Gregory Price]

When numa balancing is enabled with demotion, vmscan will call
migrate_pages when shrinking LRUs.  Successful demotions will
cause node vmstat numbers to double-decrement, leading to an
imbalanced page count.  The result is dmesg output like such:

$ cat /proc/sys/vm/stat_refresh

[77383.088417] vmstat_refresh: nr_isolated_anon -103212
[77383.088417] vmstat_refresh: nr_isolated_file -899642

This negative value may impact compaction and reclaim throttling.

The double-decrement occurs in the migrate_pages path:

caller to shrink_folio_list decrements the count
  shrink_folio_list
    demote_folio_list
      migrate_pages
        migrate_pages_batch
          migrate_folio_move
            migrate_folio_done
              mod_node_page_state(-ve) <- second decrement

This path happens for SUCCESSFUL migrations, not failures. Typically
callers to migrate_pages are required to handle putback/accounting for
failures, but this is already handled in the shrink code.

When accounting for migrations, instead do not decrement the count
when the migration reason is MR_DEMOTION. As of v6.11, this demotion
logic is the only source of MR_DEMOTION.

Signed-off-by: Gregory Price <gourry@gourry.net>
Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
Cc: stable@vger.kernel.org
---
 mm/migrate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 923ea80ba744..e3aac274cf16 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
 	 * not accounted to NR_ISOLATED_*. They can be recognized
 	 * as __folio_test_movable
 	 */
-	if (likely(!__folio_test_movable(src)))
+	if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
 		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
 				    folio_is_file_lru(src), -folio_nr_pages(src));
 
-- 
2.43.0

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Huang, Ying]

Gregory Price <gourry@gourry.net> writes:

> When numa balancing is enabled with demotion, vmscan will call
> migrate_pages when shrinking LRUs.  Successful demotions will
> cause node vmstat numbers to double-decrement, leading to an
> imbalanced page count.  The result is dmesg output like such:
>
> $ cat /proc/sys/vm/stat_refresh
>
> [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> [77383.088417] vmstat_refresh: nr_isolated_file -899642
>
> This negative value may impact compaction and reclaim throttling.
>
> The double-decrement occurs in the migrate_pages path:
>
> caller to shrink_folio_list decrements the count
>   shrink_folio_list
>     demote_folio_list
>       migrate_pages
>         migrate_pages_batch
>           migrate_folio_move
>             migrate_folio_done
>               mod_node_page_state(-ve) <- second decrement
>
> This path happens for SUCCESSFUL migrations, not failures. Typically
> callers to migrate_pages are required to handle putback/accounting for
> failures, but this is already handled in the shrink code.
>
> When accounting for migrations, instead do not decrement the count
> when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> logic is the only source of MR_DEMOTION.
>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> Cc: stable@vger.kernel.org
> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 923ea80ba744..e3aac274cf16 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
>  	 * not accounted to NR_ISOLATED_*. They can be recognized
>  	 * as __folio_test_movable
>  	 */
> -	if (likely(!__folio_test_movable(src)))
> +	if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
>  		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
>  				    folio_is_file_lru(src), -folio_nr_pages(src));

Good catch!  Thanks for doing this!  Feel free to add

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

in the future versions.

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Shakeel Butt]

On Fri, Oct 25, 2024 at 10:17:24AM GMT, Gregory Price wrote:
> When numa balancing is enabled with demotion, vmscan will call
> migrate_pages when shrinking LRUs.  Successful demotions will
> cause node vmstat numbers to double-decrement, leading to an
> imbalanced page count.  The result is dmesg output like such:
> 
> $ cat /proc/sys/vm/stat_refresh
> 
> [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> [77383.088417] vmstat_refresh: nr_isolated_file -899642
> 
> This negative value may impact compaction and reclaim throttling.
> 
> The double-decrement occurs in the migrate_pages path:
> 
> caller to shrink_folio_list decrements the count
>   shrink_folio_list
>     demote_folio_list
>       migrate_pages
>         migrate_pages_batch
>           migrate_folio_move
>             migrate_folio_done
>               mod_node_page_state(-ve) <- second decrement
> 
> This path happens for SUCCESSFUL migrations, not failures. Typically
> callers to migrate_pages are required to handle putback/accounting for
> failures, but this is already handled in the shrink code.
> 
> When accounting for migrations, instead do not decrement the count
> when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> logic is the only source of MR_DEMOTION.
> 
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> Cc: stable@vger.kernel.org

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>

This patch looks good for stable backports. For future I wonder if
instead of migrate_pages(), the caller providing the isolated folios,
manages the isolated stats (increments and decrements) similar to how
reclaim does it.

> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 923ea80ba744..e3aac274cf16 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
>  	 * not accounted to NR_ISOLATED_*. They can be recognized
>  	 * as __folio_test_movable
>  	 */
> -	if (likely(!__folio_test_movable(src)))
> +	if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
>  		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
>  				    folio_is_file_lru(src), -folio_nr_pages(src));
>  
> -- 
> 2.43.0
> 

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Gregory Price]

On Sun, Oct 27, 2024 at 10:24:10PM -0700, Shakeel Butt wrote:
> On Fri, Oct 25, 2024 at 10:17:24AM GMT, Gregory Price wrote:
> > When numa balancing is enabled with demotion, vmscan will call
> > migrate_pages when shrinking LRUs.  Successful demotions will
> > cause node vmstat numbers to double-decrement, leading to an
> > imbalanced page count.  The result is dmesg output like such:
> > 
> > $ cat /proc/sys/vm/stat_refresh
> > 
> > [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> > [77383.088417] vmstat_refresh: nr_isolated_file -899642
> > 
> > This negative value may impact compaction and reclaim throttling.
> > 
> > The double-decrement occurs in the migrate_pages path:
> > 
> > caller to shrink_folio_list decrements the count
> >   shrink_folio_list
> >     demote_folio_list
> >       migrate_pages
> >         migrate_pages_batch
> >           migrate_folio_move
> >             migrate_folio_done
> >               mod_node_page_state(-ve) <- second decrement
> > 
> > This path happens for SUCCESSFUL migrations, not failures. Typically
> > callers to migrate_pages are required to handle putback/accounting for
> > failures, but this is already handled in the shrink code.
> > 
> > When accounting for migrations, instead do not decrement the count
> > when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> > logic is the only source of MR_DEMOTION.
> > 
> > Signed-off-by: Gregory Price <gourry@gourry.net>
> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> > Cc: stable@vger.kernel.org
> 
> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> This patch looks good for stable backports. For future I wonder if
> instead of migrate_pages(), the caller providing the isolated folios,
> manages the isolated stats (increments and decrements) similar to how
> reclaim does it.
>

Note that even if you provided the folios, you'd likely still end up in
migrate_pages_batch/migrate_folio_move and subsequently the same accounting
path.  Probably there's some refactoring we can do to make the accounting
more obvious - it is very subtle here.
 
> > ---
> >  mm/migrate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 923ea80ba744..e3aac274cf16 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
> >  	 * not accounted to NR_ISOLATED_*. They can be recognized
> >  	 * as __folio_test_movable
> >  	 */
> > -	if (likely(!__folio_test_movable(src)))
> > +	if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
> >  		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
> >  				    folio_is_file_lru(src), -folio_nr_pages(src));
> >  
> > -- 
> > 2.43.0
> > 

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Huang, Ying]

Gregory Price <gourry@gourry.net> writes:

> On Sun, Oct 27, 2024 at 10:24:10PM -0700, Shakeel Butt wrote:
>> On Fri, Oct 25, 2024 at 10:17:24AM GMT, Gregory Price wrote:
>> > When numa balancing is enabled with demotion, vmscan will call
>> > migrate_pages when shrinking LRUs.  Successful demotions will
>> > cause node vmstat numbers to double-decrement, leading to an
>> > imbalanced page count.  The result is dmesg output like such:
>> > 
>> > $ cat /proc/sys/vm/stat_refresh
>> > 
>> > [77383.088417] vmstat_refresh: nr_isolated_anon -103212
>> > [77383.088417] vmstat_refresh: nr_isolated_file -899642
>> > 
>> > This negative value may impact compaction and reclaim throttling.
>> > 
>> > The double-decrement occurs in the migrate_pages path:
>> > 
>> > caller to shrink_folio_list decrements the count
>> >   shrink_folio_list
>> >     demote_folio_list
>> >       migrate_pages
>> >         migrate_pages_batch
>> >           migrate_folio_move
>> >             migrate_folio_done
>> >               mod_node_page_state(-ve) <- second decrement
>> > 
>> > This path happens for SUCCESSFUL migrations, not failures. Typically
>> > callers to migrate_pages are required to handle putback/accounting for
>> > failures, but this is already handled in the shrink code.
>> > 
>> > When accounting for migrations, instead do not decrement the count
>> > when the migration reason is MR_DEMOTION. As of v6.11, this demotion
>> > logic is the only source of MR_DEMOTION.
>> > 
>> > Signed-off-by: Gregory Price <gourry@gourry.net>
>> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
>> > Cc: stable@vger.kernel.org
>> 
>> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
>> 
>> This patch looks good for stable backports. For future I wonder if
>> instead of migrate_pages(), the caller providing the isolated folios,
>> manages the isolated stats (increments and decrements) similar to how
>> reclaim does it.
>>
>
> Note that even if you provided the folios, you'd likely still end up in
> migrate_pages_batch/migrate_folio_move and subsequently the same accounting
> path.  Probably there's some refactoring we can do to make the accounting
> more obvious - it is very subtle here.

I agree with Shakeel here.  It's better for the caller who isolates the
folios to increase and decrease the isolation counter.  And yes, some
refactoring is required.

--
Best Regards,
Huang, Ying

>> > ---
>> >  mm/migrate.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > 
>> > diff --git a/mm/migrate.c b/mm/migrate.c
>> > index 923ea80ba744..e3aac274cf16 100644
>> > --- a/mm/migrate.c
>> > +++ b/mm/migrate.c
>> > @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
>> >  	 * not accounted to NR_ISOLATED_*. They can be recognized
>> >  	 * as __folio_test_movable
>> >  	 */
>> > -	if (likely(!__folio_test_movable(src)))
>> > +	if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
>> >  		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
>> >  				    folio_is_file_lru(src), -folio_nr_pages(src));
>> >  
>> > -- 
>> > 2.43.0
>> > 

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Davidlohr Bueso]

On Fri, 25 Oct 2024, Gregory Price wrote:

>When numa balancing is enabled with demotion, vmscan will call
>migrate_pages when shrinking LRUs.  Successful demotions will
>cause node vmstat numbers to double-decrement, leading to an
>imbalanced page count.  The result is dmesg output like such:
>
>$ cat /proc/sys/vm/stat_refresh
>
>[77383.088417] vmstat_refresh: nr_isolated_anon -103212
>[77383.088417] vmstat_refresh: nr_isolated_file -899642
>
>This negative value may impact compaction and reclaim throttling.
>
>The double-decrement occurs in the migrate_pages path:
>
>caller to shrink_folio_list decrements the count
>  shrink_folio_list
>    demote_folio_list
>      migrate_pages
>        migrate_pages_batch
>          migrate_folio_move
>            migrate_folio_done
>              mod_node_page_state(-ve) <- second decrement
>
>This path happens for SUCCESSFUL migrations, not failures. Typically
>callers to migrate_pages are required to handle putback/accounting for
>failures, but this is already handled in the shrink code.
>
>When accounting for migrations, instead do not decrement the count
>when the migration reason is MR_DEMOTION. As of v6.11, this demotion
>logic is the only source of MR_DEMOTION.
>
>Signed-off-by: Gregory Price <gourry@gourry.net>
>Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
>Cc: stable@vger.kernel.org

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Yang Shi]

On Fri, Oct 25, 2024 at 7:17 AM Gregory Price <gourry@gourry.net> wrote:
>
> When numa balancing is enabled with demotion, vmscan will call
> migrate_pages when shrinking LRUs.  Successful demotions will
> cause node vmstat numbers to double-decrement, leading to an
> imbalanced page count.  The result is dmesg output like such:
>
> $ cat /proc/sys/vm/stat_refresh
>
> [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> [77383.088417] vmstat_refresh: nr_isolated_file -899642
>
> This negative value may impact compaction and reclaim throttling.
>
> The double-decrement occurs in the migrate_pages path:
>
> caller to shrink_folio_list decrements the count
>   shrink_folio_list
>     demote_folio_list
>       migrate_pages
>         migrate_pages_batch
>           migrate_folio_move
>             migrate_folio_done
>               mod_node_page_state(-ve) <- second decrement
>
> This path happens for SUCCESSFUL migrations, not failures. Typically
> callers to migrate_pages are required to handle putback/accounting for
> failures, but this is already handled in the shrink code.

AFAIK, MGLRU doesn't dec/inc this counter, so it is not
double-decrement for MGLRU. Maybe "imbalance update" is better?
Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the
commit log too.

>
> When accounting for migrations, instead do not decrement the count
> when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> logic is the only source of MR_DEMOTION.
>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> Cc: stable@vger.kernel.org

Thanks for catching this. Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 923ea80ba744..e3aac274cf16 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
>          * not accounted to NR_ISOLATED_*. They can be recognized
>          * as __folio_test_movable
>          */
> -       if (likely(!__folio_test_movable(src)))
> +       if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
>                 mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
>                                     folio_is_file_lru(src), -folio_nr_pages(src));
>
> --
> 2.43.0
>

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Gregory Price]

On Mon, Oct 28, 2024 at 01:45:48PM -0700, Yang Shi wrote:
> On Fri, Oct 25, 2024 at 7:17 AM Gregory Price <gourry@gourry.net> wrote:
> >
> > When numa balancing is enabled with demotion, vmscan will call
> > migrate_pages when shrinking LRUs.  Successful demotions will
> > cause node vmstat numbers to double-decrement, leading to an
> > imbalanced page count.  The result is dmesg output like such:
> >
> > $ cat /proc/sys/vm/stat_refresh
> >
> > [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> > [77383.088417] vmstat_refresh: nr_isolated_file -899642
> >
> > This negative value may impact compaction and reclaim throttling.
> >
> > The double-decrement occurs in the migrate_pages path:
> >
> > caller to shrink_folio_list decrements the count
> >   shrink_folio_list
> >     demote_folio_list
> >       migrate_pages
> >         migrate_pages_batch
> >           migrate_folio_move
> >             migrate_folio_done
> >               mod_node_page_state(-ve) <- second decrement
> >
> > This path happens for SUCCESSFUL migrations, not failures. Typically
> > callers to migrate_pages are required to handle putback/accounting for
> > failures, but this is already handled in the shrink code.
> 
> AFAIK, MGLRU doesn't dec/inc this counter, so it is not
> double-decrement for MGLRU. Maybe "imbalance update" is better?
> Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the
> commit log too.
>

Gotcha, so yeah saying it's an imbalance fix is more accurate.

So more accurate changelog is:


[PATCH] vmscan,migrate: fix page count imbalance on node stats when demoting pages

When numa balancing is enabled with demotion, vmscan will call
migrate_pages when shrinking LRUs.  migrate_pages will decrement the
the node's isolated page count, leading to an imbalanced count when
invoked from (MG)LRU code.

The result is dmesg output like such:

$ cat /proc/sys/vm/stat_refresh

[77383.088417] vmstat_refresh: nr_isolated_anon -103212
[77383.088417] vmstat_refresh: nr_isolated_file -899642

This negative value may impact compaction and reclaim throttling.

The following path produces the decrement:

shrink_folio_list
  demote_folio_list
    migrate_pages
      migrate_pages_batch
        migrate_folio_move
          migrate_folio_done
            mod_node_page_state(-ve) <- decrement

This path happens for SUCCESSFUL migrations, not failures. Typically
callers to migrate_pages are required to handle putback/accounting for
failures, but this is already handled in the shrink code.

When accounting for migrations, instead do not decrement the count
when the migration reason is MR_DEMOTION. As of v6.11, this demotion
logic is the only source of MR_DEMOTION.


> >
> > Signed-off-by: Gregory Price <gourry@gourry.net>
> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> > Cc: stable@vger.kernel.org
> 
> Thanks for catching this. Reviewed-by: Yang Shi <shy828301@gmail.com>
> 
> > ---
> >  mm/migrate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 923ea80ba744..e3aac274cf16 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
> >          * not accounted to NR_ISOLATED_*. They can be recognized
> >          * as __folio_test_movable
> >          */
> > -       if (likely(!__folio_test_movable(src)))
> > +       if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
> >                 mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
> >                                     folio_is_file_lru(src), -folio_nr_pages(src));
> >
> > --
> > 2.43.0
> >

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Huang, Ying]

Gregory Price <gourry@gourry.net> writes:

> On Mon, Oct 28, 2024 at 01:45:48PM -0700, Yang Shi wrote:
>> On Fri, Oct 25, 2024 at 7:17 AM Gregory Price <gourry@gourry.net> wrote:
>> >
>> > When numa balancing is enabled with demotion, vmscan will call
>> > migrate_pages when shrinking LRUs.  Successful demotions will
>> > cause node vmstat numbers to double-decrement, leading to an
>> > imbalanced page count.  The result is dmesg output like such:
>> >
>> > $ cat /proc/sys/vm/stat_refresh
>> >
>> > [77383.088417] vmstat_refresh: nr_isolated_anon -103212
>> > [77383.088417] vmstat_refresh: nr_isolated_file -899642
>> >
>> > This negative value may impact compaction and reclaim throttling.
>> >
>> > The double-decrement occurs in the migrate_pages path:
>> >
>> > caller to shrink_folio_list decrements the count
>> >   shrink_folio_list
>> >     demote_folio_list
>> >       migrate_pages
>> >         migrate_pages_batch
>> >           migrate_folio_move
>> >             migrate_folio_done
>> >               mod_node_page_state(-ve) <- second decrement
>> >
>> > This path happens for SUCCESSFUL migrations, not failures. Typically
>> > callers to migrate_pages are required to handle putback/accounting for
>> > failures, but this is already handled in the shrink code.
>> 
>> AFAIK, MGLRU doesn't dec/inc this counter, so it is not
>> double-decrement for MGLRU. Maybe "imbalance update" is better?
>> Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the
>> commit log too.
>>
>
> Gotcha, so yeah saying it's an imbalance fix is more accurate.
>
> So more accurate changelog is:
>
>
> [PATCH] vmscan,migrate: fix page count imbalance on node stats when demoting pages
>
> When numa balancing is enabled with demotion, vmscan will call
> migrate_pages when shrinking LRUs.  migrate_pages will decrement the
> the node's isolated page count, leading to an imbalanced count when
> invoked from (MG)LRU code.
>
> The result is dmesg output like such:
>
> $ cat /proc/sys/vm/stat_refresh
>
> [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> [77383.088417] vmstat_refresh: nr_isolated_file -899642

> This negative value may impact compaction and reclaim throttling.
>
> The following path produces the decrement:
>
> shrink_folio_list
>   demote_folio_list
>     migrate_pages
>       migrate_pages_batch
>         migrate_folio_move
>           migrate_folio_done
>             mod_node_page_state(-ve) <- decrement

I think that it may be better to mention the different behavior of LRU
and MGLRU.  But that's not a big deal, change it again only if you think
it's necessary.

--
Best Regards,
Huang, Ying

> This path happens for SUCCESSFUL migrations, not failures. Typically
> callers to migrate_pages are required to handle putback/accounting for
> failures, but this is already handled in the shrink code.
>
> When accounting for migrations, instead do not decrement the count
> when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> logic is the only source of MR_DEMOTION.
>
>
>> >
>> > Signed-off-by: Gregory Price <gourry@gourry.net>
>> > Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
>> > Cc: stable@vger.kernel.org
>> 
>> Thanks for catching this. Reviewed-by: Yang Shi <shy828301@gmail.com>
>> 
>> > ---
>> >  mm/migrate.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/mm/migrate.c b/mm/migrate.c
>> > index 923ea80ba744..e3aac274cf16 100644
>> > --- a/mm/migrate.c
>> > +++ b/mm/migrate.c
>> > @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
>> >          * not accounted to NR_ISOLATED_*. They can be recognized
>> >          * as __folio_test_movable
>> >          */
>> > -       if (likely(!__folio_test_movable(src)))
>> > +       if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
>> >                 mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
>> >                                     folio_is_file_lru(src), -folio_nr_pages(src));
>> >
>> > --
>> > 2.43.0
>> >

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Gregory Price]

On Tue, Oct 29, 2024 at 08:34:34AM +0800, Huang, Ying wrote:
> Gregory Price <gourry@gourry.net> writes:
> 
> > On Mon, Oct 28, 2024 at 01:45:48PM -0700, Yang Shi wrote:
> >> On Fri, Oct 25, 2024 at 7:17 AM Gregory Price <gourry@gourry.net> wrote:
> >> >
> >> > This path happens for SUCCESSFUL migrations, not failures. Typically
> >> > callers to migrate_pages are required to handle putback/accounting for
> >> > failures, but this is already handled in the shrink code.
> >> 
> >> AFAIK, MGLRU doesn't dec/inc this counter, so it is not
> >> double-decrement for MGLRU. Maybe "imbalance update" is better?
> >> Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the
> >> commit log too.
> >>
> >
> > Gotcha, so yeah saying it's an imbalance fix is more accurate.
> >
> > So more accurate changelog is:
... 
> 
> I think that it may be better to mention the different behavior of LRU
> and MGLRU.  But that's not a big deal, change it again only if you think
> it's necessary.
>

The behavior isn't really different. It's either way migrate_pages decrements
when it shouldn't going through the shink code - and both LRU and MGLRU go
through the same code.  That LRU does an inc/dec pair is irrelevant - neither
should do the decrement in the migrate path.

~Gregory

Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Oscar Salvador]

On Fri, Oct 25, 2024 at 10:17:24AM -0400, Gregory Price wrote:
> When numa balancing is enabled with demotion, vmscan will call
> migrate_pages when shrinking LRUs.  Successful demotions will
> cause node vmstat numbers to double-decrement, leading to an
> imbalanced page count.  The result is dmesg output like such:
> 
> $ cat /proc/sys/vm/stat_refresh
> 
> [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> [77383.088417] vmstat_refresh: nr_isolated_file -899642
> 
> This negative value may impact compaction and reclaim throttling.
> 
> The double-decrement occurs in the migrate_pages path:
> 
> caller to shrink_folio_list decrements the count
>   shrink_folio_list
>     demote_folio_list
>       migrate_pages
>         migrate_pages_batch
>           migrate_folio_move
>             migrate_folio_done
>               mod_node_page_state(-ve) <- second decrement
> 
> This path happens for SUCCESSFUL migrations, not failures. Typically
> callers to migrate_pages are required to handle putback/accounting for
> failures, but this is already handled in the shrink code.
> 
> When accounting for migrations, instead do not decrement the count
> when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> logic is the only source of MR_DEMOTION.
> 
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> Cc: stable@vger.kernel.org

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs