linux-kernel - Re: [PATCH v4 06/16] x86/virt/tdx: Improve PAMT refcounts allocation for sparse memory

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Message-ID: <irqkfods52iut7se552qo6b5o4qidtmghcdosdxmbytvpyphpi@ol5wuaoydaab>
Date: Thu, 27 Nov 2025 15:57:44 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, 
	"Huang, Kai" <kai.huang@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, 
	"Hansen, Dave" <dave.hansen@...el.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, 
	"Wu, Binbin" <binbin.wu@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"mingo@...hat.com" <mingo@...hat.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "Annapurve, Vishal" <vannapurve@...gle.com>, 
	"Gao, Chao" <chao.gao@...el.com>, "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v4 06/16] x86/virt/tdx: Improve PAMT refcounts allocation
 for sparse memory

On Wed, Nov 26, 2025 at 08:47:19PM +0000, Edgecombe, Rick P wrote:
> On Tue, 2025-11-25 at 11:15 +0800, Binbin Wu wrote:
> > On 11/21/2025 8:51 AM, Rick Edgecombe wrote:
> > [...]
> > > +
> > > +/* Unmap a page from the PAMT refcount vmalloc region */
> > > +static int pamt_refcount_depopulate(pte_t *pte, unsigned long addr, void *data)
> > > +{
> > > +	struct page *page;
> > > +	pte_t entry;
> > > +
> > > +	spin_lock(&init_mm.page_table_lock);
> > > +
> > > +	entry = ptep_get(pte);
> > > +	/* refcount allocation is sparse, may not be populated */
> > 
> > Not sure this comment about "sparse" is accurate since this function is called via
> > apply_to_existing_page_range().
> > 
> > And the check for not present just for sanity check?
> 
> Yes, I don't see what that comment is referring to. But we do need it, because
> hypothetically the refcount mapping could have failed halfway. So we will have
> pte_none()s for the ranges that didn't get populated. I'll use:
> 
> /* Refcount mapping could have failed part way, handle aborted mappings. */

It is possible that we can have holes in physical address space between
0 and max_pfn. You need the check even outside of "failed halfway"
scenario.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov