2024-02-06 08:48:53

by Muhammad Usama Anjum

Subject: [PATCH] mm/migrate: preserve exact soft-dirty state

From: Paul Gofman <[email protected]>

pte_mkdirty() sets both _PAGE_DIRTY and _PAGE_SOFT_DIRTY bits. The
_PAGE_SOFT_DIRTY can get set even if it wasn't set on original page
before migration. This makes non-soft-dirty pages soft-dirty just
because of migration/compaction. Clear the _PAGE_SOFT_DIRTY flag if
it wasn't set on original page.

By definition of soft-dirty feature, there can be spurious soft-dirty
pages because of kernel's internal activity such as VMA merging or
migration/compaction. This patch is eliminating the spurious soft-dirty
pages because of migration/compaction.

Cc: Michał Mirosław <[email protected]>
Cc: Andrei Vagin <[email protected]>
Signed-off-by: Paul Gofman <[email protected]>
Signed-off-by: Muhammad Usama Anjum <[email protected]>
---
mm/migrate.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index cc9f2bcd73b4..05d6ca437321 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -211,14 +211,17 @@ static bool remove_migration_pte(struct folio *folio,
folio_get(folio);
pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
old_pte = ptep_get(pvmw.pte);
- if (pte_swp_soft_dirty(old_pte))
- pte = pte_mksoft_dirty(pte);

entry = pte_to_swp_entry(old_pte);
if (!is_migration_entry_young(entry))
pte = pte_mkold(pte);
if (folio_test_dirty(folio) && is_migration_entry_dirty(entry))
pte = pte_mkdirty(pte);
+ if (pte_swp_soft_dirty(old_pte))
+ pte = pte_mksoft_dirty(pte);
+ else
+ pte = pte_clear_soft_dirty(pte);
+
if (is_writable_migration_entry(entry))
pte = pte_mkwrite(pte, vma);
else if (pte_swp_uffd_wp(old_pte))
--
2.42.0

2024-02-09 20:30:19

by Andrei Vagin

Subject: Re: [PATCH] mm/migrate: preserve exact soft-dirty state

On Tue, Feb 6, 2024 at 12:48 AM Muhammad Usama Anjum
<[email protected]> wrote:
>
> From: Paul Gofman <[email protected]>
>
> pte_mkdirty() sets both _PAGE_DIRTY and _PAGE_SOFT_DIRTY bits. The
> _PAGE_SOFT_DIRTY can get set even if it wasn't set on original page
> before migration. This makes non-soft-dirty pages soft-dirty just
> because of migration/compaction. Clear the _PAGE_SOFT_DIRTY flag if
> it wasn't set on original page.
>
> By definition of soft-dirty feature, there can be spurious soft-dirty
> pages because of kernel's internal activity such as VMA merging or
> migration/compaction. This patch is eliminating the spurious soft-dirty
> pages because of migration/compaction.
>
> Cc: Michał Mirosław <[email protected]>
> Cc: Andrei Vagin <[email protected]>

Acked-by: Andrei Vagin <[email protected]>

> Signed-off-by: Paul Gofman <[email protected]>
> Signed-off-by: Muhammad Usama Anjum <[email protected]>
> ---
> mm/migrate.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index cc9f2bcd73b4..05d6ca437321 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -211,14 +211,17 @@ static bool remove_migration_pte(struct folio *folio,
> folio_get(folio);
> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
> old_pte = ptep_get(pvmw.pte);
> - if (pte_swp_soft_dirty(old_pte))
> - pte = pte_mksoft_dirty(pte);
>
> entry = pte_to_swp_entry(old_pte);
> if (!is_migration_entry_young(entry))
> pte = pte_mkold(pte);
> if (folio_test_dirty(folio) && is_migration_entry_dirty(entry))
> pte = pte_mkdirty(pte);
> + if (pte_swp_soft_dirty(old_pte))
> + pte = pte_mksoft_dirty(pte);
> + else
> + pte = pte_clear_soft_dirty(pte);
> +
> if (is_writable_migration_entry(entry))
> pte = pte_mkwrite(pte, vma);
> else if (pte_swp_uffd_wp(old_pte))
> --
> 2.42.0
>