Re: [PATCH v3 04/11] mm: zswap: add range lookup for large-folio swapin

[Yosry Ahmed <yosry@kernel.org>]

On Fri, Jul 03, 2026 at 10:38:21AM -0700, Usama Arif wrote:
> From: Alexandre Ghiti <alexghiti@fb.com>
> 
> A large folio reaches zswap_load() only when the caller expects the
> whole range to be on disk. Zswap still stores large folios as
> independent order-0 entries, so reconstructing a large folio from
> zswap entries would risk returning partially initialized data.
> 
> Teach zswap_load() to scan the covered range. If no slot is in zswap,
> return -ENOENT so swap_read_folio() reads the backing device. If any
> slot is still in zswap, fail the large-folio read so the caller can
> fall back to per-page swapin.
> 
> Add zswap_range_has_entry() so PMD swap-entry consumers can make the
> same range decision before attempting PMD-order swapin.
> 
> Signed-off-by: Alexandre Ghiti <alexghiti@fb.com>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

I found this by accident, please CC me on zswap changes :)

> ---
>  include/linux/zswap.h |  7 +++++++
>  mm/zswap.c            | 46 +++++++++++++++++++++++++++++++++----------
>  2 files changed, 43 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 30c193a1207e..de10aa528597 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>  void zswap_folio_swapin(struct folio *folio);
>  bool zswap_is_enabled(void);
>  bool zswap_never_enabled(void);
> +bool zswap_range_has_entry(swp_entry_t entry, unsigned int nr);
>  #else
>  
>  struct zswap_lruvec_state {};
> @@ -69,6 +70,12 @@ static inline bool zswap_never_enabled(void)
>  	return true;
>  }
>  
> +static inline bool zswap_range_has_entry(swp_entry_t entry,
> +					 unsigned int nr)
> +{
> +	return false;
> +}
> +
>  #endif
>  
>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b5a17ea20237..89dd88a5223f 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1559,6 +1559,27 @@ bool zswap_store(struct folio *folio)
>  	return ret;
>  }
>  
> +/**
> + * zswap_range_has_entry() - is any slot in [entry, entry + nr) in zswap?
> + * @entry: base swap entry of the range
> + * @nr: number of contiguous slots to check
> + */

Maybe zswap_is_present() as suggested here:
https://lore.kernel.org/linux-mm/CAO9r8zNni28A+9BiDgDTBRuvGTKnqH91kpbpjjeftfnKSeSOJg@mail.gmail.com/.

IIUC this interface can be used for the same purpose, we just need to
pass nr=1.

> +bool zswap_range_has_entry(swp_entry_t entry, unsigned int nr)
> +{
> +	pgoff_t offset = swp_offset(entry);
> +	XA_STATE(xas, swap_zswap_tree(entry), offset);
> +	bool found;
> +
> +	if (!nr || zswap_never_enabled())
> +		return false;
> +
> +	rcu_read_lock();
> +	found = !!xas_find(&xas, offset + nr - 1);
> +	rcu_read_unlock();

Why not xa_find()? It seems to handle RCU locking as well as retrying.

> +
> +	return found;
> +}
> +
>  /**
>   * zswap_load() - load a folio from zswap
>   * @folio: folio to load
> @@ -1571,10 +1592,9 @@ bool zswap_store(struct folio *folio)
>   *  NOT marked up-to-date, so that an IO error is emitted (e.g. do_swap_page()
>   *  will SIGBUS).
>   *
> - *  -EINVAL: if the swapped out content was in zswap, but the page belongs
> - *  to a large folio, which is not supported by zswap. The folio is unlocked,
> - *  but NOT marked up-to-date, so that an IO error is emitted (e.g.
> - *  do_swap_page() will SIGBUS).
> + *  -EIO: if a slot in a large-folio range is unexpectedly still in zswap.
> + *  The folio is unlocked, but NOT marked up-to-date, so that an IO
> + *  error is emitted (e.g. do_swap_page() will SIGBUS).
>   *
>   *  -ENOENT: if the swapped out content was not in zswap. The folio remains
>   *  locked on return.
> @@ -1593,13 +1613,19 @@ int zswap_load(struct folio *folio)
>  		return -ENOENT;
>  
>  	/*
> -	 * Large folios should not be swapped in while zswap is being used, as
> -	 * they are not properly handled. Zswap does not properly load large
> -	 * folios, and a large folio may only be partially in zswap.
> +	 * A large folio reaches zswap_load() only when its whole range is
> +	 * expected to be on disk: PMD swap-entry consumers split before
> +	 * calling into PMD-order swapin whenever any slot is still in zswap.
> +	 * Confirm the range is entirely absent from zswap and return -ENOENT
> +	 * so the caller reads it from disk; if a slot is unexpectedly still in
> +	 * zswap, fail the read rather than return partially-initialized data.
>  	 */
> -	if (WARN_ON_ONCE(folio_test_large(folio))) {
> -		folio_unlock(folio);
> -		return -EINVAL;
> +	if (folio_test_large(folio)) {
> +		if (zswap_range_has_entry(swp, folio_nr_pages(folio))) {
> +			folio_unlock(folio);
> +			return -EIO;
> +		}
> +		return -ENOENT;
>  	}
>  
>  	entry = xa_load(tree, offset);
> -- 
> 2.53.0-Meta
> 
>