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

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Hi,

Thank you so much for reviewing.

On 2/2/23 3:51 AM, Peter Xu wrote:
> 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.
Added. Thanks

> 
>>  	 */
>>  #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?
Okay. I'll add some documentation.

> 
> Thanks,
> 

-- 
BR,
Muhammad Usama Anjum