Re: [PATCH v3 1/3] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()

[Peter Xu]

On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote:
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> 
> For now, folio_move_anon_rmap() was only used to move a folio to a
> different anon_vma after fork(), whereby the root anon_vma stayed
> unchanged. For that, it was sufficient to hold the folio lock when
> calling folio_move_anon_rmap().
> 
> However, we want to make use of folio_move_anon_rmap() to move folios
> between VMAs that have a different root anon_vma. As folio_referenced()
> performs an RMAP walk without holding the folio lock but only holding the
> anon_vma in read mode, holding the folio lock is insufficient.
> 
> When moving to an anon_vma with a different root anon_vma, we'll have to
> hold both, the folio lock and the anon_vma lock in write mode.
> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to
> read-lock the anon_vma, we have to re-check if the mapping was changed
> in the meantime. If that was the case, we have to retry.
> 
> Note that folio_move_anon_rmap() must only be called if the anon page is
> exclusive to a process, and must not be called on KSM folios.
> 
> This is a preparation for UFFDIO_MOVE, which will hold the folio lock,
> the anon_vma lock in write mode, and the mmap_lock in read mode.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  mm/rmap.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c1f11c9dbe61..f9ddc50269d2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
>  	struct anon_vma *root_anon_vma;
>  	unsigned long anon_mapping;
>  
> +retry:
>  	rcu_read_lock();
> +retry_under_rcu:
>  	anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
>  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>  		goto out;
> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
>  	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
>  	root_anon_vma = READ_ONCE(anon_vma->root);
>  	if (down_read_trylock(&root_anon_vma->rwsem)) {
> +		/*
> +		 * folio_move_anon_rmap() might have changed the anon_vma as we
> +		 * might not hold the folio lock here.
> +		 */
> +		if (unlikely((unsigned long)READ_ONCE(folio->mapping) !=
> +			     anon_mapping)) {
> +			up_read(&root_anon_vma->rwsem);
> +			goto retry_under_rcu;

Is adding this specific label worthwhile?  How about rcu unlock and goto
retry (then it'll also be clear that we won't hold rcu read lock for
unpredictable time)?

> +		}
> +
>  		/*
>  		 * If the folio is still mapped, then this anon_vma is still
>  		 * its anon_vma, and holding the mutex ensures that it will

-- 
Peter Xu