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

[Usama Arif <usama.arif@linux.dev>]

On 06/07/2026 22:36, Yosry Ahmed wrote:
> 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 :)
> 

Sorry about this! I will CC you in all future revisions of this series.
>> ---
>>  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.

Yes, this sounds better, will change in next revision.

> 
>> +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.
> 

Ack, will change in next revision, Thanks!

>> +
>> +	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
>>
>>