Re: [PATCH RFC 05/17] userfaultfd: retry copying with locks dropped in mfill_atomic_pte_copy()

[Peter Xu]

Hi, Mike,

On Tue, Jan 27, 2026 at 09:29:24PM +0200, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
> 
> Implementation of UFFDIO_COPY for anonymous memory might fail to copy
> data data from userspace buffer when the destination VMA is locked
> (either with mm_lock or with per-VMA lock).
> 
> In that case, mfill_atomic() releases the locks, retries copying the
> data with locks dropped and then re-locks the destination VMA and
> re-establishes PMD.
> 
> Since this retry-reget dance is only relevant for UFFDIO_COPY and it
> never happens for other UFFDIO_ operations, make it a part of
> mfill_atomic_pte_copy() that actually implements UFFDIO_COPY for
> anonymous memory.
> 
> shmem implementation will be updated later and the loop in
> mfill_atomic() will be adjusted afterwards.

Thanks for the refactoring.  Looks good to me in general, only some
nitpicks inline.

> 
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> ---
>  mm/userfaultfd.c | 70 +++++++++++++++++++++++++++++++-----------------
>  1 file changed, 46 insertions(+), 24 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 45d8f04aaf4f..01a2b898fa40 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -404,35 +404,57 @@ static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr)
>  	return ret;
>  }
>  
> +static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio)
> +{
> +	unsigned long src_addr = state->src_addr;
> +	void *kaddr;
> +	int err;
> +
> +	/* retry copying with mm_lock dropped */
> +	mfill_put_vma(state);
> +
> +	kaddr = kmap_local_folio(folio, 0);
> +	err = copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE);
> +	kunmap_local(kaddr);
> +	if (unlikely(err))
> +		return -EFAULT;
> +
> +	flush_dcache_folio(folio);
> +
> +	/* reget VMA and PMD, they could change underneath us */
> +	err = mfill_get_vma(state);
> +	if (err)
> +		return err;
> +
> +	err = mfill_get_pmd(state);
> +	if (err)
> +		return err;
> +
> +	return 0;
> +}
> +
>  static int mfill_atomic_pte_copy(struct mfill_state *state)
>  {
> -	struct vm_area_struct *dst_vma = state->vma;
>  	unsigned long dst_addr = state->dst_addr;
>  	unsigned long src_addr = state->src_addr;
>  	uffd_flags_t flags = state->flags;
> -	pmd_t *dst_pmd = state->pmd;
>  	struct folio *folio;
>  	int ret;
>  
> -	if (!state->folio) {
> -		ret = -ENOMEM;
> -		folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, dst_vma,
> -					dst_addr);
> -		if (!folio)
> -			goto out;
> +	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, state->vma, dst_addr);
> +	if (!folio)
> +		return -ENOMEM;
>  
> -		ret = mfill_copy_folio_locked(folio, src_addr);
> +	ret = -ENOMEM;
> +	if (mem_cgroup_charge(folio, state->vma->vm_mm, GFP_KERNEL))
> +		goto out_release;
>  
> +	ret = mfill_copy_folio_locked(folio, src_addr);
> +	if (unlikely(ret)) {
>  		/* fallback to copy_from_user outside mmap_lock */
> -		if (unlikely(ret)) {
> -			ret = -ENOENT;
> -			state->folio = folio;
> -			/* don't free the page */
> -			goto out;
> -		}
> -	} else {
> -		folio = state->folio;
> -		state->folio = NULL;
> +		ret = mfill_copy_folio_retry(state, folio);

Yes, I agree this should work and should avoid the previous ENOENT
processing that might be hard to follow.  It'll move the complexity into
mfill_state though (e.g., now it's unknown on the vma lock state after this
function returns..), but I guess it's fine.

> +		if (ret)
> +			goto out_release;
>  	}
>  
>  	/*
> @@ -442,17 +464,16 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
>  	 */
>  	__folio_mark_uptodate(folio);

Since success path should make sure vma lock held when reaching here, but
now with mfill_copy_folio_retry()'s presence it's not as clear as before,
maybe we add an assertion for that here before installing ptes?  No strong
feelings.

>  
> -	ret = -ENOMEM;
> -	if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
> -		goto out_release;
> -
> -	ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
> +	ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
>  				       &folio->page, true, flags);
>  	if (ret)
>  		goto out_release;
>  out:
>  	return ret;
>  out_release:
> +	/* Don't return -ENOENT so that our caller won't retry */
> +	if (ret == -ENOENT)
> +		ret = -EFAULT;

I recall the code removed is the only path that can return ENOENT?  Then
maybe this line isn't needed?

>  	folio_put(folio);
>  	goto out;
>  }
> @@ -907,7 +928,8 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  			break;
>  	}
>  
> -	mfill_put_vma(&state);
> +	if (state.vma)

I wonder if we should move this check into mfill_put_vma() directly, it
might be overlooked if we'll put_vma in other paths otherwise.

> +		mfill_put_vma(&state);
>  out:
>  	if (state.folio)
>  		folio_put(state.folio);
> -- 
> 2.51.0
> 

-- 
Peter Xu