Re: [RFC PATCH v2 3/7] mm, swap: support physical swap as a vswap backend

[Yosry Ahmed]

On Fri, Jun 12, 2026 at 12:37:34PM -0700, Nhat Pham wrote:
> Add physical swap as a backend for the virtual swap layer.
> 
> With physical swap backing, vswap can allocate a physical slot on
> demand when needed: as a fallback for zswap_store failures, or as
> the destination for zswap writeback.
> 
> Each vswap entry's physical slot is tracked via a Pointer-tagged
> swap_table entry on the physical cluster (rmap back to the vswap
> entry).
> 
> Suggested-by: Kairui Song <kasong@xxxxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
[..]
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 466f8a182716..5daff7a25f67 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -993,6 +993,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>  	struct folio *folio;
>  	struct mempolicy *mpol;
>  	struct swap_info_struct *si;
> +	swp_entry_t phys = {};
>  	int ret = 0;
>  
>  	/* try to allocate swap cache folio */
> @@ -1000,16 +1001,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>  	if (!si)
>  		return -EEXIST;
>  
> -	/*
> -	 * Vswap entries have no physical backing - writeback would fail
> -	 * and SIGBUS the caller. Bail before we waste a swap-cache folio
> -	 * allocation.
> -	 */
> -	if (si->flags & SWP_VSWAP) {
> -		put_swap_device(si);
> -		return -EINVAL;
> -	}
> -
>  	mpol = get_task_policy(current);
>  	folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol,
>  				       NO_INTERLEAVE_INDEX);
> @@ -1028,40 +1019,78 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>  	/*
>  	 * folio is locked, and the swapcache is now secured against
>  	 * concurrent swapping to and from the slot, and concurrent
> -	 * swapoff so we can safely dereference the zswap tree here.
> -	 * Verify that the swap entry hasn't been invalidated and recycled
> -	 * behind our backs, to avoid overwriting a new swap folio with
> -	 * old compressed data. Only when this is successful can the entry
> -	 * be dereferenced.
> +	 * swapoff so we can safely dereference the zswap tree (or vswap
> +	 * vtable) here. Verify that the swap entry hasn't been
> +	 * invalidated and recycled behind our backs, to avoid overwriting
> +	 * a new swap folio with old compressed data. Only when this is
> +	 * successful can the entry be dereferenced.
>  	 */
> -	tree = swap_zswap_tree(swpentry);
> -	if (entry != xa_load(tree, offset)) {
> -		ret = -ENOMEM;
> -		goto out;
> +	if (swap_is_vswap(si)) {
> +		if (entry != vswap_zswap_load(swpentry)) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}
> +		/*
> +		 * Allocate physical backing BEFORE decompress - if it fails,
> +		 * no wasted work. folio_realloc_swap sets vtable to PHYS,
> +		 * overwriting ZSWAP - the old entry pointer is only held
> +		 * by the caller now.
> +		 */
> +		phys = folio_realloc_swap(folio);
> +		if (!phys.val) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}

I didn't look through the rest of the series, but are there use cases
for calling folio_realloc_swap() without calling vswap_zswap_load()
first? I wonder if the realloc_swap API should take the swpentry
directly and do the load within? Something like
vswap_alloc_phys(swpentry, folio)?