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Tue, 21 Jul 2026 10:10:18 -0700 (PDT) Date: Tue, 21 Jul 2026 10:10:16 -0700 From: Vishal Moola To: "Lorenzo Stoakes (ARM)" Cc: stable@vger.kernel.org, Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , "Mike Rapoport (Microsoft)" , Jason Gunthorpe , Lu Baolu , Andrew Morton , David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Kiryl Shutsemau , iommu@lists.linux.dev, Kevin Tian Subject: Re: [PATCH v2] x86/mm/pat: allocate split page tables as kernel page tables Message-ID: References: <20260721-fix-cpa-kernel-pagetables-v2-1-2b255deed710@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260721-fix-cpa-kernel-pagetables-v2-1-2b255deed710@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D75FB80008 X-Stat-Signature: xhm8ed6qekhsrs5q1tz8m3ancum9hh9c X-HE-Tag: 1784653820-232431 X-HE-Meta: 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 OGreurfi 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jul 21, 2026 at 01:14:52PM +0100, Lorenzo Stoakes (ARM) wrote: > When splitting a large page in CPA in __split_large_page() we allocate a > PTE directly without going through the standard page table allocation > routines such as pte_alloc_one_kernel(). > > This means the page table constructor is never called nor is the page table > marked as a kernel page table. > > The former results in the folio associated with the page table not being > marked as a page table (__pagetable_ctor() is never called thus neither is > __folio_set_pgtable()) nor are statistics updated to reflect > it (lruvec_stat_add_folio() is never called). > > The latter issue of failing to mark the page table as a kernel page > table (ptdesc_set_kernel() is never called) is far more problematic. > > Since commit 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page > tables") kernel page table freeing has been batched and since the > subsequent commit e37d5a2d60a3 ("iommu/sva: invalidate stale IOTLB entries > for kernel address space") IOTLB cache entries for kernel page tables have > been invalidated upon being freed. > > Since split page tables are freed without this invalidation, the IOTLB can > contain stale entries for them. > > Resolve the issue by using the ordinary PTE allocation API at split time. > > This results in these kernel page tables invoking a page table constructor, > and thus requires a page table destructor. > > Since we cannot assume one is always present (early allocated direct map > page tables are not marked as such), we conditionally call > pagetable_dtor_free() if the PG_table folio flag for the ptdesc is set, > otherwise we free the page table via pagetable_free(). > > Regardless of which path is taken page tables marked as kernel page tables, > which now includes split page tables, take the correct route through > pagetable_free_kernel(). > > There is a user-visible side effect in that split page tables will appear > in nr_page_table_pages in /proc/vmstat (as do other kernel page tables > allocated after early boot), however this is a positive change. > > This issue started being markedly problematic after commit > 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") so > choose this as the Fixes target. > > Fixes: 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") > Cc: stable@vger.kernel.org > Signed-off-by: Lorenzo Stoakes (ARM) Acked-by: Vishal Moola