linux-kernel - Re: [PATCH v13 04/26] KVM: guest_memfd: Fix PTR_ERR() handling in __kvm_gmem_get

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Date: Fri, 19 Apr 2024 18:17:46 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Michael Roth <michael.roth@....com>
Cc: David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org, 
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com, ardb@...nel.org, 
	seanjc@...gle.com, vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org, 
	dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com, 
	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com, 
	rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de, 
	vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com, 
	sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com, 
	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com, 
	pankaj.gupta@....com, liam.merwick@...cle.com
Subject: Re: [PATCH v13 04/26] KVM: guest_memfd: Fix PTR_ERR() handling in __kvm_gmem_get_pfn()

On Fri, Apr 19, 2024 at 5:11 PM Michael Roth <michael.roth@....com> wrote:
>
> On Fri, Apr 19, 2024 at 02:58:43PM +0200, David Hildenbrand wrote:
> > On 18.04.24 21:41, Michael Roth wrote:
> > > kvm_gmem_get_folio() may return a PTR_ERR() rather than just NULL. In
> > > particular, for cases where EEXISTS is returned when FGP_CREAT_ONLY
> > > flag is used. Handle this properly in __kvm_gmem_get_pfn().
> > >
> > > Signed-off-by: Michael Roth <michael.roth@....com>
> > > ---
> > >   virt/kvm/guest_memfd.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > > index ccf22e44f387..9d7c6a70c547 100644
> > > --- a/virt/kvm/guest_memfd.c
> > > +++ b/virt/kvm/guest_memfd.c
> > > @@ -580,8 +580,8 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
> > >     }
> > >     folio = kvm_gmem_get_folio(file_inode(file), index, prepare);
> > > -   if (!folio)
> > > -           return -ENOMEM;
> > > +   if (IS_ERR_OR_NULL(folio))
> > > +           return folio ? PTR_ERR(folio) : -ENOMEM;
> >
> > Will it even return NULL?  Staring at other filemap_grab_folio() users, they
> > all check for IS_ERR().
>
> Looks like the NULL case is handled with PTR_ERR(-ENOENT), so IS_ERR()
> would be sufficient. I think in the past kvm_gmem_get_folio() itself
> would return NULL in some cases, but as of commit 2b01b7e994e95 that's
> no longer the case.
>
> I'll fix this up to expect only PTR_ERR() when I re-spin v14, and also
> address the other kvm_gmem_get_folio() / __filemap_get_folio() call
> sites.
>
> >
> > >     if (folio_test_hwpoison(folio)) {
> > >             r = -EHWPOISON;
> >
> > Do we have a Fixes: tag?
>
> Fixes: 2b01b7e994e95 ("KVM: guest_memfd: pass error up from filemap_grab_folio")

I'll squash it so when you rebase on the new kvm-coco-queue it will go
away. Thanks to both!

Paolo