Re: [PATCH v4 04/11] mm: zswap: add range lookup for large-folio swapin

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

On 13/07/2026 17:02, Yosry Ahmed wrote:
>>  /**
>>   * zswap_load() - load a folio from zswap
>>   * @folio: folio to load
>> @@ -1571,10 +1588,9 @@ 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).
> 
> Just curious, why the change from -EINVAL to -EIO here? Does it matter
> in practice or is it just a matter of personal taste? :)

The caller only checks for != -ENOENT so no functional difference here. I viewed
it as a change in semantic, -EINVAL meant that you passed a large folio, which
wasn't supported to -EIO meaning the request is valid but zswap cannot safely
complete a PMD-sized read from per-page compressed state.

I dont want to speak for Alexandre, will let him comment on this properly :)
But above is what I thought of when I got the patch from Alex.

> 
>>   *
>>   *  -ENOENT: if the swapped out content was not in zswap. The folio remains
>>   *  locked on return.
>> @@ -1593,13 +1609,19 @@ 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))) {
>> +                       folio_unlock(folio);
>> +                       return -EIO;
>> +               }
>> +               return -ENOENT;
>>         }
>>
>>         entry = xa_load(tree, offset);
>> --
>> 2.53.0-Meta
>>