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

[Kiryl Shutsemau]

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