linux-kernel - Re: [PATCH v1 2/5] s390/uv: convert gmap_make

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Message-ID: <272c0f71-e817-44a6-8798-0a1535427ba9@redhat.com>
Date: Thu, 11 Apr 2024 11:09:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-s390@...r.kernel.org, kvm@...r.kernel.org,
 Matthew Wilcox <willy@...radead.org>, Heiko Carstens <hca@...ux.ibm.com>,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Thomas Huth <thuth@...hat.com>
Subject: Re: [PATCH v1 2/5] s390/uv: convert gmap_make_secure() to work on
 folios

[...]

>> +	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