linux-kernel - Re: [PATCH v3 2/6] KVM: guest_memfd: Remove partial hugepage handling from kvm_gmem

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Message-ID: <aWk9PusYNW0iADuD@google.com>
Date: Thu, 15 Jan 2026 11:17:18 -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 2/6] KVM: guest_memfd: Remove partial hugepage handling
 from kvm_gmem_populate()

On Thu, Jan 08, 2026, Michael Roth wrote:
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index fdaea3422c30..9dafa44838fe 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -151,6 +151,15 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>  					 mapping_gfp_mask(inode->i_mapping), policy);
>  	mpol_cond_put(policy);
>  
> +	/*
> +	 * External interfaces like kvm_gmem_get_pfn() support dealing
> +	 * with hugepages to a degree, but internally, guest_memfd currently
> +	 * assumes that all folios are order-0 and handling would need
> +	 * to be updated for anything otherwise (e.g. page-clearing
> +	 * operations).
> +	 */
> +	WARN_ON_ONCE(folio_order(folio));

Gah, this is buggy.  __filemap_get_folio_mpol() can return an ERR_PTR().  If that
happens, this WARN will dereference garbage and explode.

And of course I find it _just_ after sending thank yous, *sigh*.

I'll squash to this (after testing):

	WARN_ON_ONCE(!IS_ERR(folio) && folio_order(folio));

avoiding a few emails isn't worth having a lurking bug.