Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation

[Peter Xu <peterx@redhat.com>]

Suren,

Sorry to review so late.

On Fri, Sep 22, 2023 at 06:31:44PM -0700, Suren Baghdasaryan wrote:
> diff --git a/mm/rmap.c b/mm/rmap.c
> index ec7f8e6c9e48..c1ebbd23fa61 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -542,6 +542,7 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
>  	struct anon_vma *root_anon_vma;
>  	unsigned long anon_mapping;
>  
> +repeat:
>  	rcu_read_lock();
>  	anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
>  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
> @@ -586,6 +587,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
>  	rcu_read_unlock();
>  	anon_vma_lock_read(anon_vma);
>  
> +	/*
> +	 * Check if UFFDIO_REMAP changed the anon_vma. This is needed
> +	 * because we don't assume the folio was locked.
> +	 */
> +	if (unlikely((unsigned long) READ_ONCE(folio->mapping) !=
> +		     anon_mapping)) {
> +		anon_vma_unlock_read(anon_vma);
> +		put_anon_vma(anon_vma);
> +		anon_vma = NULL;
> +		goto repeat;
> +	}

We have an open-coded fast path above this:

	if (down_read_trylock(&root_anon_vma->rwsem)) {
		/*
		 * If the folio is still mapped, then this anon_vma is still
		 * its anon_vma, and holding the mutex ensures that it will
		 * not go away, see anon_vma_free().
		 */
		if (!folio_mapped(folio)) {
			up_read(&root_anon_vma->rwsem);
			anon_vma = NULL;
		}
		goto out;
	}

Would that also need such check?

> +
>  	if (atomic_dec_and_test(&anon_vma->refcount)) {
>  		/*
>  		 * Oops, we held the last refcount, release the lock
> -- 
> 2.42.0.515.g380fc7ccd1-goog
> 

-- 
Peter Xu