Re: [PATCH v3 04/14] mm/rmap: Break COW PTE in rmap walking

[Barry Song <baohua@kernel.org>]

On Tue, Dec 20, 2022 at 8:25 PM Chih-En Lin <shiyn.lin@gmail.com> wrote:
>
> Some of the features (unmap, migrate, device exclusive, mkclean, etc)
> might modify the pte entry via rmap. Add a new page vma mapped walk
> flag, PVMW_BREAK_COW_PTE, to indicate the rmap walking to break COW PTE.
>
> Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
> ---
>  include/linux/rmap.h |  2 ++
>  mm/migrate.c         |  3 ++-
>  mm/page_vma_mapped.c |  2 ++
>  mm/rmap.c            | 12 +++++++-----
>  mm/vmscan.c          |  7 ++++++-
>  5 files changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index bd3504d11b155..d0f07e5519736 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -368,6 +368,8 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
>  #define PVMW_SYNC              (1 << 0)
>  /* Look for migration entries rather than present PTEs */
>  #define PVMW_MIGRATION         (1 << 1)
> +/* Break COW-ed PTE during walking */
> +#define PVMW_BREAK_COW_PTE     (1 << 2)
>
>  struct page_vma_mapped_walk {
>         unsigned long pfn;
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dff333593a8ae..a4be7e04c9b09 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -174,7 +174,8 @@ void putback_movable_pages(struct list_head *l)
>  static bool remove_migration_pte(struct folio *folio,
>                 struct vm_area_struct *vma, unsigned long addr, void *old)
>  {
> -       DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> +       DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr,
> +                             PVMW_SYNC | PVMW_MIGRATION | PVMW_BREAK_COW_PTE);
>
>         while (page_vma_mapped_walk(&pvmw)) {
>                 rmap_t rmap_flags = RMAP_NONE;
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 93e13fc17d3cb..5dfc9236dc505 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -251,6 +251,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>                         step_forward(pvmw, PMD_SIZE);
>                         continue;
>                 }
> +               if (pvmw->flags & PVMW_BREAK_COW_PTE)
> +                       break_cow_pte(vma, pvmw->pmd, pvmw->address);
>                 if (!map_pte(pvmw))
>                         goto next_pte;
>  this_pte:
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2ec925e5fa6a9..b1b7dcbd498be 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -807,7 +807,8 @@ static bool folio_referenced_one(struct folio *folio,
>                 struct vm_area_struct *vma, unsigned long address, void *arg)
>  {
>         struct folio_referenced_arg *pra = arg;
> -       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> +       /* it will clear the entry, so we should break COW PTE. */
> +       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);

what do you mean by breaking cow pte? in memory reclamation case, we are only
checking and clearing page referenced bit in pte, do we really need to
break cow?

>         int referenced = 0;
>
>         while (page_vma_mapped_walk(&pvmw)) {
> @@ -1012,7 +1013,8 @@ static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
>  static bool page_mkclean_one(struct folio *folio, struct vm_area_struct *vma,
>                              unsigned long address, void *arg)
>  {
> -       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_SYNC);
> +       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address,
> +                             PVMW_SYNC | PVMW_BREAK_COW_PTE);
>         int *cleaned = arg;
>
>         *cleaned += page_vma_mkclean_one(&pvmw);
> @@ -1471,7 +1473,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>                      unsigned long address, void *arg)
>  {
>         struct mm_struct *mm = vma->vm_mm;
> -       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> +       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
>         pte_t pteval;
>         struct page *subpage;
>         bool anon_exclusive, ret = true;
> @@ -1842,7 +1844,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>                      unsigned long address, void *arg)
>  {
>         struct mm_struct *mm = vma->vm_mm;
> -       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> +       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
>         pte_t pteval;
>         struct page *subpage;
>         bool anon_exclusive, ret = true;
> @@ -2195,7 +2197,7 @@ static bool page_make_device_exclusive_one(struct folio *folio,
>                 struct vm_area_struct *vma, unsigned long address, void *priv)
>  {
>         struct mm_struct *mm = vma->vm_mm;
> -       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> +       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
>         struct make_exclusive_args *args = priv;
>         pte_t pteval;
>         struct page *subpage;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 026199c047e0e..980d2056adfd1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1781,6 +1781,10 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>                         }
>                 }
>
> +               /*
> +                * Break COW PTE since checking the reference
> +                * of folio might modify the PTE.
> +                */
>                 if (!ignore_references)
>                         references = folio_check_references(folio, sc);
>
> @@ -1864,7 +1868,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>
>                 /*
>                  * The folio is mapped into the page tables of one or more
> -                * processes. Try to unmap it here.
> +                * processes. Try to unmap it here. Also, since it will write
> +                * to the page tables, break COW PTE if they are.
>                  */
>                 if (folio_mapped(folio)) {
>                         enum ttu_flags flags = TTU_BATCH_FLUSH;
> --
> 2.37.3
>

Thanks
Barry