Re: [PATCH v1 2/5] s390/uv: convert gmap_make_secure() to work on folios

[David Hildenbrand <david@redhat.com>]

[...]

>> +	if (rc == -E2BIG) {
>> +		/*
>> +		 * Splitting might fail with -EBUSY due to unexpected folio
>> +		 * references, just like make_folio_secure(). So handle it
>> +		 * ahead of time without the PTL being held.
>> +		 */
>> +		folio_lock(folio);
>> +		rc = split_folio(folio);
> 
> if split_folio returns -EAGAIN...
> 
>> +		folio_unlock(folio);
>> +		folio_put(folio);
>> +	}
>> +
>>   	if (rc == -EAGAIN) {
> 
> ... we will not skip this ...
> 
>>   		/*
>>   		 * If we are here because the UVC returned busy or partial
>>   		 * completion, this is just a useless check, but it is safe.
>>   		 */
>> -		wait_on_page_writeback(page);
>> -		put_page(page);
>> +		folio_wait_writeback(folio);
>> +		folio_put(folio);
> 
> ... and we will do one folio_put() too many
> 
>>   	} else if (rc == -EBUSY) {
>>   		/*
>>   		 * If we have tried a local drain and the page refcount
> 
> are we sure that split_folio() can never return -EAGAIN now and in the
> future too?

Yes, and in contrast to documentation, that can actually happen! The 
documentation is even wrong: we return -EAGAIN if there are unexpected 
folio references (e.g., pinned), thanks for raising that.

> 
> maybe just change it to  } else if (...   ?


I think I'll make it all clearer by handling split_folio() return values 
separately.

-- 
Cheers,

David / dhildenb