Re: [PATCH v4] mm: improve mprotect(R|W) efficiency on pages referenced once
From: Peter Xu <peterx@redhat.com>
Date: 2021-05-27 21:41:29
Peter, On Thu, May 27, 2021 at 12:04:53PM -0700, Peter Collingbourne wrote: [...]
+static bool may_avoid_write_fault(pte_t pte, struct vm_area_struct *vma,
+ unsigned long cp_flags)
+{
+ if (!(cp_flags & MM_CP_DIRTY_ACCT)) {
+ if (!(vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE)))
+ return false;
+
+ if (page_count(pte_page(pte)) != 1)
+ return false;
+ }
Can we make MM_CP_DIRTY_ACCT still in charge? IIUC that won't affect your use
case, something like:
/* Never apply trick if MM_CP_DIRTY_ACCT not set */
if (!(cp_flags & MM_CP_DIRTY_ACCT))
return false;
The thing is that's really what MM_CP_DIRTY_ACCT is about, imho (as its name
shows). Say, we should start to count on the dirty bit for applying the write
bit only if that flag set. With above, I think we can drop the pte_uffd_wp()
check below because uffd_wp never applies MM_CP_DIRTY_ACCT when do
change_protection().
Thanks,
quoted hunk ↗ jump to hunk
+ + if (!pte_dirty(pte)) + return false; + + if (!pte_soft_dirty(pte) && (vma->vm_flags & VM_SOFTDIRTY)) + return false; + + if (pte_uffd_wp(pte)) + return false; + + return true; +} + static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr, unsigned long end, pgprot_t newprot, unsigned long cp_flags)@@ -43,7 +66,6 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, spinlock_t *ptl; unsigned long pages = 0; int target_node = NUMA_NO_NODE; - bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT; bool prot_numa = cp_flags & MM_CP_PROT_NUMA; bool uffd_wp = cp_flags & MM_CP_UFFD_WP; bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;@@ -132,11 +154,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, } /* Avoid taking write faults for known dirty pages */ - if (dirty_accountable && pte_dirty(ptent) && - (pte_soft_dirty(ptent) || - !(vma->vm_flags & VM_SOFTDIRTY))) { + if (may_avoid_write_fault(ptent, vma, cp_flags)) ptent = pte_mkwrite(ptent); - } ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent); pages++; } else if (is_swap_pte(oldpte)) {-- 2.32.0.rc0.204.g9fa02ecfa5-goog
-- Peter Xu