Received-SPF: pass (google.com: domain of linux-kernel+bounces-139129-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Date: Wed, 10 Apr 2024 19:32:10 +0200
From: Claudio Imbrenda <imbrenda@linux.ibm.com>
To: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org,
        Heiko
 Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
        Alexander
 Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger
 <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Janosch
 Frank <frankja@linux.ibm.com>,
        Gerald Schaefer
 <gerald.schaefer@linux.ibm.com>,
        Thomas Huth <thuth@redhat.com>
Subject: Re: [PATCH v1 2/5] s390/uv: convert gmap_make_secure() to work on
 folios
Message-ID: <20240410193210.61f3e069@p-imbrenda>
In-Reply-To: <67557c5b-afd8-4578-a00d-6750accc1026@redhat.com>
References: <20240404163642.1125529-1-david@redhat.com>
	<20240404163642.1125529-3-david@redhat.com>
	<Zg9wNKTu4JxGXrHs@casper.infradead.org>
	<67557c5b-afd8-4578-a00d-6750accc1026@redhat.com>
Organization: IBM
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit

On Fri, 5 Apr 2024 09:09:30 +0200
David Hildenbrand <david@redhat.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