Re: [PATCH v9 1/3] userfaultfd: Add UFFD WP Async support

[Peter Xu <peterx@redhat.com>]

On Tue, Jan 31, 2023 at 01:32:55PM +0500, Muhammad Usama Anjum wrote:
> Add new WP Async mode (UFFD_FEATURE_WP_ASYNC) which resolves the page
> faults on its own. It can be used to track that which pages have been
> written-to from the time the pages were write-protected. It is very
> efficient way to track the changes as uffd is by nature pte/pmd based.
> 
> UFFD synchronous WP sends the page faults to the userspace where the
> pages which have been written-to can be tracked. But it is not efficient.
> This is why this asynchronous version is being added. After setting the
> WP Async, the pages which have been written to can be found in the pagemap
> file or information can be obtained from the PAGEMAP_IOCTL.
> 
> Suggested-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
> Changes in v9:
> - Correct the fault resolution with code contributed by Peter
> 
> Changes in v7:
> - Remove UFFDIO_WRITEPROTECT_MODE_ASYNC_WP and add UFFD_FEATURE_WP_ASYNC
> - Handle automatic page fault resolution in better way (thanks to Peter)
> 
> update to wp async
> ---
>  fs/userfaultfd.c                 | 11 +++++++++++
>  include/linux/userfaultfd_k.h    |  6 ++++++
>  include/uapi/linux/userfaultfd.h |  8 +++++++-
>  mm/memory.c                      | 23 ++++++++++++++++++++---
>  4 files changed, 44 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 15a5bf765d43..c17835a0e842 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1867,6 +1867,10 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
>  	mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
>  	mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;
>  
> +	/* The unprotection is not supported if in async WP mode */
> +	if (!mode_wp && (ctx->features & UFFD_FEATURE_WP_ASYNC))
> +		return -EINVAL;
> +
>  	if (mode_wp && mode_dontwake)
>  		return -EINVAL;
>  
> @@ -1950,6 +1954,13 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
>  	return ret;
>  }
>  
> +int userfaultfd_wp_async(struct vm_area_struct *vma)
> +{
> +	struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
> +
> +	return (ctx && (ctx->features & UFFD_FEATURE_WP_ASYNC));
> +}
> +
>  static inline unsigned int uffd_ctx_features(__u64 user_features)
>  {
>  	/*
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 9df0b9a762cc..94dcb4dc1b4a 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -179,6 +179,7 @@ extern int userfaultfd_unmap_prep(struct mm_struct *mm, unsigned long start,
>  				  unsigned long end, struct list_head *uf);
>  extern void userfaultfd_unmap_complete(struct mm_struct *mm,
>  				       struct list_head *uf);
> +extern int userfaultfd_wp_async(struct vm_area_struct *vma);
>  
>  #else /* CONFIG_USERFAULTFD */
>  
> @@ -274,6 +275,11 @@ static inline bool uffd_disable_fault_around(struct vm_area_struct *vma)
>  	return false;
>  }
>  
> +static inline int userfaultfd_wp_async(struct vm_area_struct *vma)
> +{
> +	return false;
> +}
> +
>  #endif /* CONFIG_USERFAULTFD */
>  
>  static inline bool pte_marker_entry_uffd_wp(swp_entry_t entry)
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 005e5e306266..f4252ef40071 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -38,7 +38,8 @@
>  			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
>  			   UFFD_FEATURE_MINOR_SHMEM |		\
>  			   UFFD_FEATURE_EXACT_ADDRESS |		\
> -			   UFFD_FEATURE_WP_HUGETLBFS_SHMEM)
> +			   UFFD_FEATURE_WP_HUGETLBFS_SHMEM |	\
> +			   UFFD_FEATURE_WP_ASYNC)
>  #define UFFD_API_IOCTLS				\
>  	((__u64)1 << _UFFDIO_REGISTER |		\
>  	 (__u64)1 << _UFFDIO_UNREGISTER |	\
> @@ -203,6 +204,10 @@ struct uffdio_api {
>  	 *
>  	 * UFFD_FEATURE_WP_HUGETLBFS_SHMEM indicates that userfaultfd
>  	 * write-protection mode is supported on both shmem and hugetlbfs.
> +	 *
> +	 * UFFD_FEATURE_WP_ASYNC indicates that userfaultfd write-protection
> +	 * asynchronous mode is supported in which the write fault is automatically
> +	 * resolved and write-protection is un-set.

Please mention a few other things:

  - It only supports anon and shmem (so hugetlb is not supported)

  - It will only take effect when any vma is registered with wr-protection
    mode.  Otherwise the flag will be ignored.

In userfaultfd_register(), we need to fail the ioctl if anyone tries to
register any hugetlb vma with this new flag set.

>  	 */
>  #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
>  #define UFFD_FEATURE_EVENT_FORK			(1<<1)
> @@ -217,6 +222,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_MINOR_SHMEM		(1<<10)
>  #define UFFD_FEATURE_EXACT_ADDRESS		(1<<11)
>  #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM		(1<<12)
> +#define UFFD_FEATURE_WP_ASYNC			(1<<13)
>  	__u64 features;
>  
>  	__u64 ioctls;
> diff --git a/mm/memory.c b/mm/memory.c
> index 4000e9f017e0..04843e35550e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3351,8 +3351,21 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  
>  	if (likely(!unshare)) {
>  		if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> -			pte_unmap_unlock(vmf->pte, vmf->ptl);
> -			return handle_userfault(vmf, VM_UFFD_WP);
> +			if (userfaultfd_wp_async(vma)) {
> +				/*
> +				 * Nothing needed (cache flush, TLB invalidations,
> +				 * etc.) because we're only removing the uffd-wp bit,
> +				 * which is completely invisible to the user.
> +				 */
> +				pte_t pte = pte_clear_uffd_wp(*vmf->pte);
> +
> +				set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> +				/* Update this to be prepared for following up CoW handling */
> +				vmf->orig_pte = pte;
> +			} else {
> +				pte_unmap_unlock(vmf->pte, vmf->ptl);
> +				return handle_userfault(vmf, VM_UFFD_WP);
> +			}
>  		}
>  
>  		/*
> @@ -4812,8 +4825,11 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
>  
>  	if (vma_is_anonymous(vmf->vma)) {
>  		if (likely(!unshare) &&
> -		    userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd))
> +		    userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd)) {
> +			if (userfaultfd_wp_async(vmf->vma))
> +				goto split_and_return;
>  			return handle_userfault(vmf, VM_UFFD_WP);
> +		}
>  		return do_huge_pmd_wp_page(vmf);
>  	}
>  
> @@ -4825,6 +4841,7 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
>  		}
>  	}
>  
> +split_and_return:

The "and_return" is superfluous, IMHO.  Just make it "split"?

>  	/* COW or write-notify handled on pte level: split pmd. */
>  	__split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);

Would you also update Documentation/admin-guide/mm/userfaultfd.rst in the
same patch?

Thanks,

-- 
Peter Xu