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mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()
Patch series "mm: uffd-wp + change_protection() cleanups". Cleanup page protection handling in uffd-wp when calling change_protection() and improve unprotecting uffd=wp in private mappings, trying to set PTEs writable again if possible just like we do during mprotect() when upgrading write permissions. Make the change_protection() interface harder to get wrong :) I consider both pages primarily cleanups, although patch #1 fixes a corner case with uffd-wp and softdirty tracking for shmem. @Peter, please let me know if we should flag patch #1 as pure cleanup -- I have no idea how important softdirty tracking on shmem is. This patch (of 2): uffd_wp_range() currently calculates page protection manually using vm_get_page_prot(). This will ignore any other reason for active writenotify: one mechanism applicable to shmem is softdirty tracking. For example, the following sequence 1) Write to mapped shmem page 2) Clear softdirty 3) Register uffd-wp covering the mapped page 4) Unregister uffd-wp covering the mapped page 5) Write to page again will not set the modified page softdirty, because uffd_wp_range() will ignore that writenotify is required for softdirty tracking and simply map the page writable again using change_protection(). Similarly, instead of unregistering, protecting followed by un-protecting the page using uffd-wp would result in the same situation. Now that we enable writenotify whenever enabling uffd-wp on a VMA, vma->vm_page_prot will already properly reflect our requirements: the default is to write-protect all PTEs. However, for shared mappings we would now not remap the PTEs writable if possible when unprotecting, just like for private mappings (COW). To compensate, set MM_CP_TRY_CHANGE_WRITABLE just like mprotect() does to try mapping individual PTEs writable. For private mappings, this change implies that we will now always try setting PTEs writable when un-protecting, just like when upgrading write permissions using mprotect(), which is an improvement. For shared mappings, we will only set PTEs writable if can_change_pte_writable()/can_change_pmd_writable() indicates that it's ok. For ordinary shmem, this will be the case when PTEs are dirty, which should usually be the case -- otherwise we could special-case shmem in can_change_pte_writable()/can_change_pmd_writable() easily, because shmem itself doesn't require writenotify. Note that hugetlb does not yet implement MM_CP_TRY_CHANGE_WRITABLE, so we won't try setting PTEs writable when unprotecting or when unregistering uffd-wp. This can be added later on top by implementing MM_CP_TRY_CHANGE_WRITABLE. While commitffd0579396
("userfaultfd: wp: support write protection for userfault vma range") introduced that code, it should only be applicable to uffd-wp on shared mappings -- shmem (hugetlb does not support softdirty tracking). I don't think this corner cases justifies to cc stable. Let's just handle it correctly and prepare for change_protection() cleanups. [david@redhat.com: o need for additional harmless checks if we're wr-protecting either way] Link: https://lkml.kernel.org/r/71412742-a71f-9c74-865f-773ad83db7a5@redhat.com Link: https://lkml.kernel.org/r/20221223155616.297723-1-david@redhat.com Link: https://lkml.kernel.org/r/20221223155616.297723-2-david@redhat.com Fixes:b1f9e87686
("mm/uffd: enable write protection for shmem & hugetlbfs") Signed-off-by: David Hildenbrand <david@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Nadav Amit <nadav.amit@gmail.com> Cc: Peter Xu <peterx@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -713,17 +713,25 @@ ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
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void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma,
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unsigned long start, unsigned long len, bool enable_wp)
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{
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unsigned int mm_cp_flags;
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struct mmu_gather tlb;
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pgprot_t newprot;
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if (enable_wp)
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newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE));
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mm_cp_flags = MM_CP_UFFD_WP;
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else
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newprot = vm_get_page_prot(dst_vma->vm_flags);
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mm_cp_flags = MM_CP_UFFD_WP_RESOLVE;
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/*
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* vma->vm_page_prot already reflects that uffd-wp is enabled for this
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* VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed
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* to be write-protected as default whenever protection changes.
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* Try upgrading write permissions manually.
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*/
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if (!enable_wp && vma_wants_manual_pte_write_upgrade(dst_vma))
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mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
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tlb_gather_mmu(&tlb, dst_mm);
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change_protection(&tlb, dst_vma, start, start + len, newprot,
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enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE);
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change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot,
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mm_cp_flags);
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tlb_finish_mmu(&tlb);
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}
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