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

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Message-ID: <aWabORpkzEJygYNQ@google.com>
Date: Tue, 13 Jan 2026 11:21:29 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Michael Roth <michael.roth@....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 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?

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;
			}
		}

		...