linux-kernel - Re: [PATCH 1/5] KVM: gmem: Don't rely on __kvm_gmem_get

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Message-ID: <20250220011211.tjgo66egqxwqjzdk@amd.com>
Date: Wed, 19 Feb 2025 19:12:11 -0600
From: Michael Roth <michael.roth@....com>
To: Tom Lendacky <thomas.lendacky@....com>
CC: <kvm@...r.kernel.org>, <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <jroedel@...e.de>, <pbonzini@...hat.com>,
	<seanjc@...gle.com>, <vbabka@...e.cz>, <amit.shah@....com>,
	<pratikrajesh.sampat@....com>, <ashish.kalra@....com>,
	<liam.merwick@...cle.com>, <david@...hat.com>, <vannapurve@...gle.com>,
	<ackerleytng@...gle.com>, <quic_eberman@...cinc.com>
Subject: Re: [PATCH 1/5] KVM: gmem: Don't rely on __kvm_gmem_get_pfn() for
 preparedness

On Wed, Jan 22, 2025 at 08:39:37AM -0600, Tom Lendacky wrote:
> On 12/12/24 00:36, Michael Roth wrote:
> > Currently __kvm_gmem_get_pfn() sets 'is_prepared' so callers can skip
> > calling kvm_gmem_prepare_folio(). However, subsequent patches will
> > introduce some locking constraints around setting/checking preparedness
> > that will require filemap_invalidate_lock*() to be held while checking
> > for preparedness. This locking could theoretically be done inside
> > __kvm_gmem_get_pfn(), or by requiring that filemap_invalidate_lock*() is
> > held while calling __kvm_gmem_get_pfn(), but that places unnecessary
> > constraints around when __kvm_gmem_get_pfn() can be called, whereas
> > callers could just as easily call kvm_gmem_is_prepared() directly.
> > 
> > So, in preparation for these locking changes, drop the 'is_prepared'
> > argument, and leave it up to callers to handle checking preparedness
> > where needed and with the proper locking constraints.
> > 
> > Signed-off-by: Michael Roth <michael.roth@....com>
> > ---
> >  virt/kvm/guest_memfd.c | 13 +++++--------
> >  1 file changed, 5 insertions(+), 8 deletions(-)
> > 
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index b69af3580bef..aa0038ddf4a4 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -773,7 +773,7 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
> >  static struct folio *__kvm_gmem_get_pfn(struct file *file,
> >  					struct kvm_memory_slot *slot,
> >  					pgoff_t index, kvm_pfn_t *pfn,
> > -					bool *is_prepared, int *max_order)
> > +					int *max_order)
> >  {
> >  	struct kvm_gmem *gmem = file->private_data;
> >  	struct folio *folio;
> > @@ -803,7 +803,6 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
> >  	if (max_order)
> >  		*max_order = 0;
> >  
> > -	*is_prepared = kvm_gmem_is_prepared(file, index, folio);
> >  	return folio;
> >  }
> >  
> > @@ -814,19 +813,18 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> >  	pgoff_t index = kvm_gmem_get_index(slot, gfn);
> >  	struct file *file = kvm_gmem_get_file(slot);
> >  	struct folio *folio;
> > -	bool is_prepared = false;
> >  	int r = 0;
> >  
> >  	if (!file)
> >  		return -EFAULT;
> >  
> > -	folio = __kvm_gmem_get_pfn(file, slot, index, pfn, &is_prepared, max_order);
> > +	folio = __kvm_gmem_get_pfn(file, slot, index, pfn, max_order);
> >  	if (IS_ERR(folio)) {
> >  		r = PTR_ERR(folio);
> >  		goto out;
> >  	}
> >  
> > -	if (!is_prepared)
> > +	if (kvm_gmem_is_prepared(file, index, folio))
> 
> Shouldn't this be !kvm_gmem_is_prepared() ?

Yes indeed. It looks like I fixed this up later, but accidentally squashed it
into PATCH #2 rather than here. Will fix for the next spin.

Thanks,

Mike

> 
> Thanks,
> Tom
> 
> >  		r = kvm_gmem_prepare_folio(kvm, file, slot, gfn, folio);
> >  
> >  	folio_unlock(folio);
> > @@ -872,7 +870,6 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
> >  		struct folio *folio;
> >  		gfn_t gfn = start_gfn + i;
> >  		pgoff_t index = kvm_gmem_get_index(slot, gfn);
> > -		bool is_prepared = false;
> >  		kvm_pfn_t pfn;
> >  
> >  		if (signal_pending(current)) {
> > @@ -880,13 +877,13 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
> >  			break;
> >  		}
> >  
> > -		folio = __kvm_gmem_get_pfn(file, slot, index, &pfn, &is_prepared, &max_order);
> > +		folio = __kvm_gmem_get_pfn(file, slot, index, &pfn, &max_order);
> >  		if (IS_ERR(folio)) {
> >  			ret = PTR_ERR(folio);
> >  			break;
> >  		}
> >  
> > -		if (is_prepared) {
> > +		if (kvm_gmem_is_prepared(file, index, folio)) {
> >  			folio_unlock(folio);
> >  			folio_put(folio);
> >  			ret = -EEXIST;
>