Re: [PATCH v2 2/5] userfaultfd, shmem: use a VMA callback to handle UFFDIO_CONTINUE

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Mike Rapoport <rppt@kernel.org> [251125 13:39]:
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> 
> When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE
> it needs to get a folio that already exists in the pagecache backing
> that VMA.
> 
> Instead of using shmem_get_folio() for that, add a get_folio() method to
> 'struct vm_operations_struct' that will return a folio if it exists in
> the VMA's pagecache at given pgoff.
> 
> Implement get_folio() method for shmem and slightly refactor
> userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support
> this new API.
> 
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  include/linux/mm.h |  9 ++++++++
>  mm/shmem.c         | 18 ++++++++++++++++
>  mm/userfaultfd.c   | 52 +++++++++++++++++++++++++++++-----------------
>  3 files changed, 60 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7c79b3369b82..c8647707d75b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -690,6 +690,15 @@ struct vm_operations_struct {
>  	struct page *(*find_normal_page)(struct vm_area_struct *vma,
>  					 unsigned long addr);
>  #endif /* CONFIG_FIND_NORMAL_PAGE */
> +#ifdef CONFIG_USERFAULTFD
> +	/*
> +	 * Called by userfault to resolve UFFDIO_CONTINUE request.
> +	 * Should return the folio found at pgoff in the VMA's pagecache if it
> +	 * exists or ERR_PTR otherwise.
> +	 * The returned folio is locked and with reference held.
> +	 */
> +	struct folio *(*get_folio)(struct inode *inode, pgoff_t pgoff);
> +#endif
>  };
>  
>  #ifdef CONFIG_NUMA_BALANCING
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 58701d14dd96..e16c7c8c3e1e 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3263,6 +3263,18 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
>  	shmem_inode_unacct_blocks(inode, 1);
>  	return ret;
>  }
> +
> +static struct folio *shmem_get_folio_noalloc(struct inode *inode, pgoff_t pgoff)
> +{
> +	struct folio *folio;
> +	int err;
> +
> +	err = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
> +	if (err)
> +		return ERR_PTR(err);
> +
> +	return folio;
> +}
>  #endif /* CONFIG_USERFAULTFD */
>  
>  #ifdef CONFIG_TMPFS
> @@ -5295,6 +5307,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
>  	.set_policy     = shmem_set_policy,
>  	.get_policy     = shmem_get_policy,
>  #endif
> +#ifdef CONFIG_USERFAULTFD
> +	.get_folio	= shmem_get_folio_noalloc,
> +#endif
>  };
>  
>  static const struct vm_operations_struct shmem_anon_vm_ops = {
> @@ -5304,6 +5319,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
>  	.set_policy     = shmem_set_policy,
>  	.get_policy     = shmem_get_policy,
>  #endif
> +#ifdef CONFIG_USERFAULTFD
> +	.get_folio	= shmem_get_folio_noalloc,
> +#endif
>  };
>  
>  int shmem_init_fs_context(struct fs_context *fc)
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 8dc964389b0d..9f0f879b603a 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -388,15 +388,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
>  	struct page *page;
>  	int ret;
>  
> -	ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
> +	folio = dst_vma->vm_ops->get_folio(inode, pgoff);
>  	/* Our caller expects us to return -EFAULT if we failed to find folio */
> -	if (ret == -ENOENT)
> -		ret = -EFAULT;
> -	if (ret)
> -		goto out;
> -	if (!folio) {
> -		ret = -EFAULT;
> -		goto out;
> +	if (IS_ERR_OR_NULL(folio)) {
> +		if (PTR_ERR(folio) == -ENOENT || !folio)
> +			return -EFAULT;
> +		return PTR_ERR(folio);
>  	}
>  
>  	page = folio_file_page(folio, pgoff);
> @@ -411,13 +408,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
>  		goto out_release;
>  
>  	folio_unlock(folio);
> -	ret = 0;
> -out:
> -	return ret;
> +	return 0;
> +
>  out_release:
>  	folio_unlock(folio);
>  	folio_put(folio);
> -	goto out;
> +	return ret;

I really like this part.

>  }
>  
>  /* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
> @@ -694,6 +690,15 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd,
>  	return err;
>  }
>  
> +static __always_inline bool vma_can_mfill_atomic(struct vm_area_struct *vma,
> +						 uffd_flags_t flags)
> +{
> +	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> +		return vma->vm_ops && vma->vm_ops->get_folio;
> +
> +	return vma_is_anonymous(vma) || vma_is_shmem(vma);
> +}
> +
>  static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  					    unsigned long dst_start,
>  					    unsigned long src_start,
> @@ -766,10 +771,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  		return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
>  					     src_start, len, flags);
>  
> -	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> -		goto out_unlock;
> -	if (!vma_is_shmem(dst_vma) &&
> -	    uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> +	if (!vma_can_mfill_atomic(dst_vma, flags))
>  		goto out_unlock;
>  
>  	while (src_addr < src_start + len) {
> @@ -1985,9 +1987,21 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
>  	if (vma->vm_flags & VM_DROPPABLE)
>  		return false;
>  
> -	if ((vm_flags & VM_UFFD_MINOR) &&
> -	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
> -		return false;
> +	if (vm_flags & VM_UFFD_MINOR) {
> +		/*
> +		 * If only MINOR mode is requested and we can request an
> +		 * existing folio from VMA's page cache, allow it
> +		 */
> +		if (vm_flags == VM_UFFD_MINOR && vma->vm_ops &&
> +		    vma->vm_ops->get_folio)
> +			return true;
> +		/*
> +		 * Only hugetlb and shmem can support MINOR mode in combination
> +		 * with other modes
> +		 */
> +		if (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))
> +			return false;
> +	}
>  
>  	/*
>  	 * If wp async enabled, and WP is the only mode enabled, allow any
> -- 
> 2.50.1
>