[PATCH v2] mm/migrate: improve mlock tracking for PTE-mapped large folios

[PATCH v2] mm/migrate: improve mlock tracking for PTE-mapped large folios

[Wandun Chen]

From: Wandun Chen <chenwandun@lixiang.com>

Commit ab521b4142aa ("mm/rmap: improve mlock tracking for large folios")
mlocks a large folio when all its pages are added in one rmap operation.
However, when a PTE-mapped large folio is migrated, remove_migration_pte()
restores PTEs individually, passing nr_pages = 1 to the rmap helpers. As a
result, PG_mlocked is not set on the destination folio.

Reclaim can restore mlock in try_to_unmap(), but Mlocked in /proc/meminfo
remains undercounted until that scan occurs.

Restore mlock once the full mapping has been restored in a locked VMA
under one page table lock, rather than waiting for try_to_unmap() to
restore the mlock state.

Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
---
 mm/migrate.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 94033c222e04..140e291d4587 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -353,6 +353,7 @@ static bool remove_migration_pte(struct folio *folio,
 {
 	struct rmap_walk_arg *rmap_walk_arg = arg;
 	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+	unsigned long nr_ptes = 0;
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
@@ -446,9 +447,16 @@ static bool remove_migration_pte(struct folio *folio,
 			else
 				folio_add_file_rmap_pte(folio, new, vma);
 			set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
+
+			nr_ptes++;
 		}
-		if (READ_ONCE(vma->vm_flags) & VM_LOCKED)
+		if (READ_ONCE(vma->vm_flags) & VM_LOCKED) {
+			if (folio_test_large(folio) &&
+			    nr_ptes == folio_nr_pages(folio) &&
+			    !(pvmw.flags & PVMW_PGTABLE_CROSSED))
+				mlock_vma_folio(folio, vma);
 			mlock_drain_local();
+		}
 
 		trace_remove_migration_pte(pvmw.address, pte_val(pte),
 					   compound_order(new));
-- 
2.43.0

Re: [PATCH v2] mm/migrate: improve mlock tracking for PTE-mapped large folios

[Wandun]

On 11/10/26 20:32, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@lixiang.com>
> 
> Commit ab521b4142aa ("mm/rmap: improve mlock tracking for large folios")
> mlocks a large folio when all its pages are added in one rmap operation.
> However, when a PTE-mapped large folio is migrated, remove_migration_pte()
> restores PTEs individually, passing nr_pages = 1 to the rmap helpers. As a
> result, PG_mlocked is not set on the destination folio.
> 
> Reclaim can restore mlock in try_to_unmap(), but Mlocked in /proc/meminfo
> remains undercounted until that scan occurs.
> 
> Restore mlock once the full mapping has been restored in a locked VMA
> under one page table lock, rather than waiting for try_to_unmap() to
> restore the mlock state.

I forgot to include the v1-to-v2 changelog:

Changes in v2:
1. Reword the subject and commit message.
2. Drop the Fixes tag.

Best regards,
Wandun

> 
> Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
> ---
>  mm/migrate.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 94033c222e04..140e291d4587 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -353,6 +353,7 @@ static bool remove_migration_pte(struct folio *folio,
>  {
>  	struct rmap_walk_arg *rmap_walk_arg = arg;
>  	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> +	unsigned long nr_ptes = 0;
>  
>  	while (page_vma_mapped_walk(&pvmw)) {
>  		rmap_t rmap_flags = RMAP_NONE;
> @@ -446,9 +447,16 @@ static bool remove_migration_pte(struct folio *folio,
>  			else
>  				folio_add_file_rmap_pte(folio, new, vma);
>  			set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
> +
> +			nr_ptes++;
>  		}
> -		if (READ_ONCE(vma->vm_flags) & VM_LOCKED)
> +		if (READ_ONCE(vma->vm_flags) & VM_LOCKED) {
> +			if (folio_test_large(folio) &&
> +			    nr_ptes == folio_nr_pages(folio) &&
> +			    !(pvmw.flags & PVMW_PGTABLE_CROSSED))
> +				mlock_vma_folio(folio, vma);
>  			mlock_drain_local();
> +		}
>  
>  		trace_remove_migration_pte(pvmw.address, pte_val(pte),
>  					   compound_order(new));

Re: [PATCH v2] mm/migrate: improve mlock tracking for PTE-mapped large folios

[David Hildenbrand (Arm)]

On 11/10/26 13:32, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@lixiang.com>
> 
> Commit ab521b4142aa ("mm/rmap: improve mlock tracking for large folios")
> mlocks a large folio when all its pages are added in one rmap operation.
> However, when a PTE-mapped large folio is migrated, remove_migration_pte()
> restores PTEs individually, passing nr_pages = 1 to the rmap helpers. As a
> result, PG_mlocked is not set on the destination folio.
> 
> Reclaim can restore mlock in try_to_unmap(), but Mlocked in /proc/meminfo
> remains undercounted until that scan occurs.
> 
> Restore mlock once the full mapping has been restored in a locked VMA
> under one page table lock, rather than waiting for try_to_unmap() to
> restore the mlock state.
> 
> Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
> ---
>  mm/migrate.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 94033c222e04..140e291d4587 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -353,6 +353,7 @@ static bool remove_migration_pte(struct folio *folio,
>  {
>  	struct rmap_walk_arg *rmap_walk_arg = arg;
>  	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> +	unsigned long nr_ptes = 0;
>  
>  	while (page_vma_mapped_walk(&pvmw)) {
>  		rmap_t rmap_flags = RMAP_NONE;
> @@ -446,9 +447,16 @@ static bool remove_migration_pte(struct folio *folio,
>  			else
>  				folio_add_file_rmap_pte(folio, new, vma);
>  			set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
> +
> +			nr_ptes++;
>  		}
> -		if (READ_ONCE(vma->vm_flags) & VM_LOCKED)
> +		if (READ_ONCE(vma->vm_flags) & VM_LOCKED) {
> +			if (folio_test_large(folio) &&
> +			    nr_ptes == folio_nr_pages(folio) &&
> +			    !(pvmw.flags & PVMW_PGTABLE_CROSSED))
> +				mlock_vma_folio(folio, vma);
>  			mlock_drain_local();
> +		}
>  
>  		trace_remove_migration_pte(pvmw.address, pte_val(pte),
>  					   compound_order(new));

I guess pte batching would be one way of handling it, but that requires more churn.

The spirit is similar as in try_to_unmap_one(), where we count the number of
ptes in one VMA. For consistency, I would call it "ptes".

@Kiryl, can you take a look?

BTW, why don't we care about that in try_to_migrate?

-- 
Cheers,

David