Re: [PATCH v5 04/11] mm: zswap: add range lookup for large-folio swapin

[Yosry Ahmed <yosry@kernel.org>]

On Wed, Jul 22, 2026 at 08:19:35AM -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.
> 
> Return -EIO rather than -EINVAL for that conflict. Large-folio loads
> are now valid requests; the error means zswap cannot safely satisfy
> the request from partial per-page compressed state, not that the
> request is unsupported. Existing callers only distinguish -ENOENT,
> so this is a semantic clarification rather than a behavioral change.
> 
> Add zswap_is_present() 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>
> ---
>  include/linux/zswap.h |  6 ++++++
>  mm/zswap.c            | 42 ++++++++++++++++++++++++++++++++----------
>  2 files changed, 38 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 30c193a1207e..cd9efcf9dec9 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_is_present(swp_entry_t entry, unsigned int nr);
>  #else
>  
>  struct zswap_lruvec_state {};
> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>  	return true;
>  }
>  
> +static inline bool zswap_is_present(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 4e76a4a87cdc..384492f1f696 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1561,6 +1561,23 @@ bool zswap_store(struct folio *folio)
>  	return ret;
>  }
>  
> +/**
> + * zswap_is_present() - is any slot in [entry, entry + nr) in zswap?
> + * @entry: base swap entry of the range
> + * @nr: number of contiguous slots to check (pass 1 for a single-slot query)
> + */
> +bool zswap_is_present(swp_entry_t entry, unsigned int nr)
> +{
> +	pgoff_t offset = swp_offset(entry);
> +	struct xarray *tree = swap_zswap_tree(entry);
> +	unsigned long index = offset;
> +
> +	if (!nr || zswap_never_enabled())
> +		return false;
> +
> +	return xa_find(tree, &index, offset + nr - 1, XA_PRESENT);
> +}
> +
>  /**
>   * zswap_load() - load a folio from zswap
>   * @folio: folio to load
> @@ -1573,10 +1590,9 @@ 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.
> @@ -1595,13 +1611,19 @@ 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_is_present(swp, folio_nr_pages(folio))) {

Is dropping the warning here intentional (for the folio_test_large() &&
zswap_is_present() case)?

> +			folio_unlock(folio);
> +			return -EIO;
> +		}
> +		return -ENOENT;
>  	}
>  
>  	entry = xa_load(tree, offset);
> -- 
> 2.53.0-Meta
>