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[67.164.59.41]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-2b832f8e987sm373629eec.18.2026.02.03.13.07.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 03 Feb 2026 13:07:44 -0800 (PST) Date: Tue, 3 Feb 2026 13:07:42 -0800 From: "Vishal Moola (Oracle)" To: Dave Hansen Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, "Mike Rapoport (Microsoft)" , akpm@linux-foundation.org, "Matthew Wilcox (Oracle)" , Dave Hansen , Andy Lutomirski , Peter Zijlstra Subject: Re: [PATCH v3 1/3] x86/mm/pat: Convert pte code to use ptdescs Message-ID: References: <20260202172005.683870-1-vishal.moola@gmail.com> <20260202172005.683870-2-vishal.moola@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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())