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    SubjectRe: [PATCH v8 1/3] mm/mmap: separate writenotify and dirty tracking logic
    On 03.05.23 00:51, Lorenzo Stoakes wrote:
    > vma_wants_writenotify() is specifically intended for setting PTE page table
    > flags, accounting for existing page table flag state and whether the
    > filesystem performs dirty tracking.
    >
    > Separate out the notions of dirty tracking and PTE write notify checking in
    > order that we can invoke the dirty tracking check from elsewhere.
    >
    > Note that this change introduces a very small duplicate check of the
    > separated out vm_ops_needs_writenotify() and vma_is_shared_writable()
    > functions. This is necessary to avoid making vma_needs_dirty_tracking()
    > needlessly complicated (e.g. passing flags or having it assume checks were
    > already performed). This is small enough that it doesn't seem too
    > egregious.
    >
    > We check to ensure the mapping is shared writable, as any GUP caller will
    > be safe - MAP_PRIVATE mappings will be CoW'd and read-only file-backed
    > shared mappings are not permitted access, even with FOLL_FORCE.
    >
    > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
    > Reviewed-by: John Hubbard <jhubbard@nvidia.com>
    > Reviewed-by: Mika Penttilä <mpenttil@redhat.com>
    > Reviewed-by: Jan Kara <jack@suse.cz>
    > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
    > ---
    > include/linux/mm.h | 1 +
    > mm/mmap.c | 53 ++++++++++++++++++++++++++++++++++------------
    > 2 files changed, 41 insertions(+), 13 deletions(-)
    >
    > diff --git a/include/linux/mm.h b/include/linux/mm.h
    > index 27ce77080c79..7b1d4e7393ef 100644
    > --- a/include/linux/mm.h
    > +++ b/include/linux/mm.h
    > @@ -2422,6 +2422,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
    > #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
    > MM_CP_UFFD_WP_RESOLVE)
    >
    > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
    > int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
    > static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
    > {
    > diff --git a/mm/mmap.c b/mm/mmap.c
    > index 5522130ae606..fa7442e44cc2 100644
    > --- a/mm/mmap.c
    > +++ b/mm/mmap.c
    > @@ -1475,6 +1475,42 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
    > }
    > #endif /* __ARCH_WANT_SYS_OLD_MMAP */
    >
    > +/* Do VMA operations imply write notify is required? */

    Nit: comment is superfluous, this is already self-documenting code.

    > +static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
    > +{
    > + return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
    > +}
    > +
    > +/* Is this VMA shared and writable? */

    Nit: dito

    > +static bool vma_is_shared_writable(struct vm_area_struct *vma)
    > +{
    > + return (vma->vm_flags & (VM_WRITE | VM_SHARED)) ==
    > + (VM_WRITE | VM_SHARED);
    > +}
    > +
    > +/*
    > + * Does this VMA require the underlying folios to have their dirty state
    > + * tracked?
    > + */

    Nit: dito

    > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
    > +{
    > + /* Only shared, writable VMAs require dirty tracking. */
    > + if (!vma_is_shared_writable(vma))
    > + return false;
    > +
    > + /* Does the filesystem need to be notified? */
    > + if (vm_ops_needs_writenotify(vma->vm_ops))
    > + return true;
    > +
    > + /* Specialty mapping? */
    > + if (vma->vm_flags & VM_PFNMAP)
    > + return false;
    > +
    > + /* Can the mapping track the dirty pages? */
    > + return vma->vm_file && vma->vm_file->f_mapping &&
    > + mapping_can_writeback(vma->vm_file->f_mapping);
    > +}
    > +
    > /*
    > * Some shared mappings will want the pages marked read-only
    > * to track write events. If so, we'll downgrade vm_page_prot
    > @@ -1483,21 +1519,18 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
    > */
    > int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
    > {
    > - vm_flags_t vm_flags = vma->vm_flags;
    > - const struct vm_operations_struct *vm_ops = vma->vm_ops;
    > -
    > /* If it was private or non-writable, the write bit is already clear */
    > - if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
    > + if (!vma_is_shared_writable(vma))
    > return 0;
    >
    > /* The backer wishes to know when pages are first written to? */
    > - if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))
    > + if (vm_ops_needs_writenotify(vma->vm_ops))
    > return 1;
    >
    > /* The open routine did something to the protections that pgprot_modify
    > * won't preserve? */
    > if (pgprot_val(vm_page_prot) !=
    > - pgprot_val(vm_pgprot_modify(vm_page_prot, vm_flags)))
    > + pgprot_val(vm_pgprot_modify(vm_page_prot, vma->vm_flags)))
    > return 0;
    >
    > /*
    > @@ -1511,13 +1544,7 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
    > if (userfaultfd_wp(vma))
    > return 1;
    >
    > - /* Specialty mapping? */
    > - if (vm_flags & VM_PFNMAP)
    > - return 0;
    > -
    > - /* Can the mapping track the dirty pages? */
    > - return vma->vm_file && vma->vm_file->f_mapping &&
    > - mapping_can_writeback(vma->vm_file->f_mapping);
    > + return vma_needs_dirty_tracking(vma);
    > }
    >
    > /*

    We now have duplicate vma_is_shared_writable() and
    vm_ops_needs_writenotify() checks ...


    Maybe move the VM_PFNMAP and "/* Can the mapping track the dirty pages?
    */" checks into a separate helper and call that from both,
    vma_wants_writenotify() and vma_needs_dirty_tracking() ?


    In any case

    Acked-by: David Hildenbrand <david@redhat.com>

    --
    Thanks,

    David / dhildenb

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