linux-kernel - Re: [PATCH v3 6/6] KVM: guest_memfd: GUP source pages prior to populating guest memory

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Message-ID: <20260113213556.rwihf3v3ek3c5kwl@amd.com>
Date: Tue, 13 Jan 2026 15:35:56 -0600
From: Michael Roth <michael.roth@....com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <kvm@...r.kernel.org>, <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <thomas.lendacky@....com>,
	<pbonzini@...hat.com>, <vbabka@...e.cz>, <ashish.kalra@....com>,
	<liam.merwick@...cle.com>, <david@...hat.com>, <vannapurve@...gle.com>,
	<ackerleytng@...gle.com>, <aik@....com>, <ira.weiny@...el.com>,
	<yan.y.zhao@...el.com>, <pankaj.gupta@....com>, Kai Huang
	<kai.huang@...el.com>
Subject: Re: [PATCH v3 6/6] KVM: guest_memfd: GUP source pages prior to
 populating guest memory

On Tue, Jan 13, 2026 at 11:21:29AM -0800, Sean Christopherson wrote:
> On Thu, Jan 08, 2026, Michael Roth wrote:
> > @@ -842,47 +881,38 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
> >  	if (!file)
> >  		return -EFAULT;
> >  
> > -	filemap_invalidate_lock(file->f_mapping);
> > -
> >  	npages = min_t(ulong, slot->npages - (start_gfn - slot->base_gfn), npages);
> >  	for (i = 0; i < npages; i++) {
> > -		struct folio *folio;
> > -		gfn_t gfn = start_gfn + i;
> > -		pgoff_t index = kvm_gmem_get_index(slot, gfn);
> > -		kvm_pfn_t pfn;
> > +		struct page *src_page = NULL;
> > +		void __user *p;
> >  
> >  		if (signal_pending(current)) {
> >  			ret = -EINTR;
> >  			break;
> >  		}
> >  
> > -		folio = __kvm_gmem_get_pfn(file, slot, index, &pfn, NULL);
> > -		if (IS_ERR(folio)) {
> > -			ret = PTR_ERR(folio);
> > -			break;
> > -		}
> > +		p = src ? src + i * PAGE_SIZE : NULL;
> >  
> > -		folio_unlock(folio);
> > +		if (p) {
> 
> Computing 'p' when src==NULL is unnecessary and makes it hard to see that gup()
> is done if and only if src!=NULL.
> 
> Anyone object to this fixup?

No objections here, and it does seem a bit more readable. Will include
this if a new version is needed.

Thanks,

Mike

> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 18ae59b92257..66afab8f08a3 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -884,17 +884,16 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
>         npages = min_t(ulong, slot->npages - (start_gfn - slot->base_gfn), npages);
>         for (i = 0; i < npages; i++) {
>                 struct page *src_page = NULL;
> -               void __user *p;
>  
>                 if (signal_pending(current)) {
>                         ret = -EINTR;
>                         break;
>                 }
>  
> -               p = src ? src + i * PAGE_SIZE : NULL;
> +               if (src) {
> +                       unsigned long uaddr = (unsigned long)src + i * PAGE_SIZE;
>  
> -               if (p) {
> -                       ret = get_user_pages_fast((unsigned long)p, 1, 0, &src_page);
> +                       ret = get_user_pages_fast(uaddr, 1, 0, &src_page);
>                         if (ret < 0)
>                                 break;
>                         if (ret != 1) {
> 
> To end up with:
> 
> 		struct page *src_page = NULL;
> 
> 		if (signal_pending(current)) {
> 			ret = -EINTR;
> 			break;
> 		}
> 
> 		if (src) {
> 			unsigned long uaddr = (unsigned long)src + i * PAGE_SIZE;
> 
> 			ret = get_user_pages_fast(uaddr, 1, 0, &src_page);
> 			if (ret < 0)
> 				break;
> 			if (ret != 1) {
> 				ret = -ENOMEM;
> 				break;
> 			}
> 		}
> 
> 		...