Re: [PATCH v8 1/3] mm/mmap: separate writenotify and dirty tracking logic

[David Hildenbrand]

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@xxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Reviewed-by: Mika Penttilä <mpenttil@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> ---
>   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@xxxxxxxxxx>

--
Thanks,

David / dhildenb