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

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Date: Wed, 10 Apr 2024 19:32:10 +0200
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org, kvm@...r.kernel.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

On Fri, 5 Apr 2024 09:09:30 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 05.04.24 05:29, Matthew Wilcox wrote:
> > On Thu, Apr 04, 2024 at 06:36:39PM +0200, David Hildenbrand wrote:  
> >> +		/* We might get PTE-mapped large folios; split them first. */
> >> +		if (folio_test_large(folio)) {
> >> +			rc = -E2BIG;  
> > 
> > We agree to this point.  I just turned this into -EINVAL.
> >   
> >>   
> >> +	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);
> >> +		folio_unlock(folio);
> >> +		folio_put(folio);
> >> +	}  
> > 
> > Ummm ... if split_folio() succeeds, aren't we going to return 0 from
> > this function, which will be interpreted as make_folio_secure() having
> > succeeded?  
> 
> I assume the code would have to handle that, because it must deal with 
> possible races that would try to convert the folio page.
> 
> But the right thing to do is
> 
> if (!rc)
> 	goto again;
> 
> after the put.

yes please