linux-kernel - Re: [PATCH v3 1/3] x86/mm/pat: Convert pte code to use ptdescs

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Message-ID: <aYJjnh6qI_bzfAo_@fedora>
Date: Tue, 3 Feb 2026 13:07:42 -0800
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
	"Mike Rapoport (Microsoft)" <rppt@...nel.org>,
	akpm@...ux-foundation.org,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/3] x86/mm/pat: Convert pte code to use ptdescs

On Tue, Feb 03, 2026 at 09:23:47AM -0800, Dave Hansen wrote:
> On 2/2/26 09:20, Vishal Moola (Oracle) wrote:
> > In order to separately allocate ptdescs from pages, we need all allocation
> > and free sites to use the appropriate functions. Convert these pte
> > allocation/free sites to use ptdescs.
> 
> Imperative voice, please.

I'll fix it.

> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 6c6eb486f7a6..f9f9d4ca8e71 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -1408,7 +1408,7 @@ static bool try_to_free_pte_page(pte_t *pte)
> >  		if (!pte_none(pte[i]))
> >  			return false;
> >  
> > -	free_page((unsigned long)pte);
> > +	pagetable_free(virt_to_ptdesc((void *)pte));
> >  	return true;
> >  }
> 
> This looks wrong to me, or at least that the API needs improvement. Most
> callers are going to have a pointer that they've been modifying. They're
> not going to have a ptdesc handy.

Yeah the API needs improvement. The initial API I wrote was very
barebones back when I didn't understand enough about arch differences
and similarities in page table implementation.

> So I think this needs to look like:
> 
> 	pagetable_free(pte);
> 
> You can convert to ptdescs internally or do whatever you want with
> ptdesc sanity checks, but the API needs to be on writeable pointers. If
> the API takes a const pointer that requires callers to cast it, I think
> the API is broken.

Your logic makes sense to me. I can add ptdesc-using-address apis.

> > @@ -1537,12 +1537,15 @@ static void unmap_pud_range(p4d_t *p4d, unsigned long start, unsigned long end)
> >  	 */
> >  }
> >  
> > -static int alloc_pte_page(pmd_t *pmd)
> > +static int alloc_pte_ptdesc(pmd_t *pmd)
> 
> Why change the name? Nobody cares what this is doing internally.
> 
> >  {
> > -	pte_t *pte = (pte_t *)get_zeroed_page(GFP_KERNEL);
> > -	if (!pte)
> > +	pte_t *pte;
> > +	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL | __GFP_ZERO, 0);
> > +
> > +	if (!ptdesc)
> >  		return -1;
> 
> This also looks wrong.
> 
> What kind of maniac is ever going to allocate page tables without
> __GFP_ZERO? __GFP_ZERO really should be a part of pagetable_alloc(),
> don't you think?

I thought the same thing... Turns out some architectures do. I didn't
question it, they might not even have good reason to do so.

Regardless, I do agree with you. I'm tempted to include
__GFP_ZERO as part of the ptdesc-using-address apis.

> > +	pte = (pte_t *) ptdesc_address(ptdesc);
> >  	set_pmd(pmd, __pmd(__pa(pte) | _KERNPG_TABLE));
> >  	return 0;
> >  }
> 
> Why is there a cast here? ptdesc_address() returns void*, no?

Yes it does.

Personally, I view casts as human hints to make implicit conversions
obvious. I didn't think it hurt readability so I left it in.

I don't have strong feelings either way, I can remove the casts. The
type is obvious enough here anyway.

> Also, if there a ptdesc_pa(), this could be:
> 
> static int alloc_pte_ptdesc(pmd_t *pmd)
> {
> 	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL, 0);
> 
> 	if (!ptdesc)
>  		return -1;
> 
>  	set_pmd(pmd, __pmd(ptdesc_pa(ptdesc) | _KERNPG_TABLE));
>  	return 0;
> }
> 
> This *should* be a very common pattern. After you allocate a page table
> page, you almost always need its physical address because it's going to
> get pointed to by other page table or hardware register.

I don't recall running into this pattern much, but I'll take a look.
It sounds sensible. If this happens far from the allocation sites, I
definitely would've missed them.

> To me, it doesn't look like the ptdesc API is very mature yet, or at
> least hasn't been expanded for ease for actual users. I don't want to
> grow its use in arch/x86 until it's a wee bit more mature.

I truly appreciate the review and comments :).

The use of struct ptdesc in cpa_collapse_large_pages() gets in the way
of short-term (stop refcounting page tables) and long term (shrinking
struct page) goals. Particularly the pagetable_free() call.

Would you be ok with taking these patches if I add these relevant apis:
1) A function that returns and address (like get_zeroed_page())
2) A function that frees by address (like free_page())