Re: [PATCH v3 04/14] mm/rmap: Break COW PTE in rmap walking
From: Barry Song
Date: Mon Dec 26 2022 - 04:41:15 EST
On Tue, Dec 20, 2022 at 8:25 PM Chih-En Lin <shiyn.lin@xxxxxxxxx> 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@xxxxxxxxx>
> ---
> 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