On Fri, Nov 20, 2020 at 02:35:55PM +0000, Will Deacon wrote:
> Since commit 0758cd830494 ("asm-generic/tlb: avoid potential double flush"),
> TLB invalidation is elided in tlb_finish_mmu() if no entries were batched
> via the tlb_remove_*() functions. Consequently, the page-table modifications
> performed by clear_refs_write() in response to a write to
> /proc/<pid>/clear_refs do not perform TLB invalidation. Although this is
> fine when simply aging the ptes, in the case of clearing the "soft-dirty"
> state we can end up with entries where pte_write() is false, yet a
> writable mapping remains in the TLB.
I don't think we need a TLB flush in this context, same reason as we
don't have one in copy_present_pte() which uses ptep_set_wrprotect()
to write-protect a src PTE.
ptep_modify_prot_start/commit() and ptep_set_wrprotect() guarantee
either the dirty bit is set (when a PTE is still writable) or a PF
happens (when a PTE has become r/o) when h/w page table walker races
with kernel that modifies a PTE using the two APIs.
> Fix this by calling tlb_remove_tlb_entry() for each entry being
> write-protected when cleating soft-dirty.
>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
> fs/proc/task_mmu.c | 18 +++++++++++-------
> 1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index cd03ab9087b0..3308292ee5c5 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1032,11 +1032,12 @@ enum clear_refs_types {
>
> struct clear_refs_private {
> enum clear_refs_types type;
> + struct mmu_gather *tlb;
> };
>
> #ifdef CONFIG_MEM_SOFT_DIRTY
> static inline void clear_soft_dirty(struct vm_area_struct *vma,
> - unsigned long addr, pte_t *pte)
> + unsigned long addr, pte_t *pte, struct mmu_gather *tlb)
> {
> /*
> * The soft-dirty tracker uses #PF-s to catch writes
> @@ -1053,6 +1054,7 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
> ptent = pte_wrprotect(old_pte);
> ptent = pte_clear_soft_dirty(ptent);
> ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
> + tlb_remove_tlb_entry(tlb, pte, addr);
> } else if (is_swap_pte(ptent)) {
> ptent = pte_swp_clear_soft_dirty(ptent);
> set_pte_at(vma->vm_mm, addr, pte, ptent);
> @@ -1060,14 +1062,14 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
> }
> #else
> static inline void clear_soft_dirty(struct vm_area_struct *vma,
> - unsigned long addr, pte_t *pte)
> + unsigned long addr, pte_t *pte, struct mmu_gather *tlb)
> {
> }
> #endif
>
> #if defined(CONFIG_MEM_SOFT_DIRTY) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
> - unsigned long addr, pmd_t *pmdp)
> + unsigned long addr, pmd_t *pmdp, struct mmu_gather *tlb)
> {
> pmd_t old, pmd = *pmdp;
>
> @@ -1081,6 +1083,7 @@ static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
>
> pmd = pmd_wrprotect(pmd);
> pmd = pmd_clear_soft_dirty(pmd);
> + tlb_remove_pmd_tlb_entry(tlb, pmdp, addr);
>
> set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
> } else if (is_migration_entry(pmd_to_swp_entry(pmd))) {
> @@ -1090,7 +1093,7 @@ static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
> }
> #else
> static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
> - unsigned long addr, pmd_t *pmdp)
> + unsigned long addr, pmd_t *pmdp, struct mmu_gather *tlb)
> {
> }
> #endif
> @@ -1107,7 +1110,7 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
> ptl = pmd_trans_huge_lock(pmd, vma);
> if (ptl) {
> if (cp->type == CLEAR_REFS_SOFT_DIRTY) {
> - clear_soft_dirty_pmd(vma, addr, pmd);
> + clear_soft_dirty_pmd(vma, addr, pmd, cp->tlb);
> goto out;
> }
>
> @@ -1133,7 +1136,7 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
> ptent = *pte;
>
> if (cp->type == CLEAR_REFS_SOFT_DIRTY) {
> - clear_soft_dirty(vma, addr, pte);
> + clear_soft_dirty(vma, addr, pte, cp->tlb);
> continue;
> }
>
> @@ -1212,7 +1215,8 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
> if (mm) {
> struct mmu_notifier_range range;
> struct clear_refs_private cp = {
> - .type = type,
> + .type = type,
> + .tlb = &tlb,
> };
>
> if (type == CLEAR_REFS_MM_HIWATER_RSS) {
> --
> 2.29.2.454.gaff20da3a2-goog
>
