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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 , stable@vger.kernel.org Subject: Re: [PATCH] x86/mm/pat: allocate split page tables as kernel page tables Message-ID: References: <20260720-fix-cpa-kernel-pagetables-v1-1-0766e782cefe@kernel.org> Precedence: bulk X-Mailing-List: stable@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, Jul 21, 2026 at 08:43:51AM +0100, Lorenzo Stoakes (ARM) wrote: > On Mon, Jul 20, 2026 at 01:03:57PM -0700, Vishal Moola wrote: > > On Mon, Jul 20, 2026 at 01:01:00PM -0700, Vishal Moola wrote: > > > On Mon, Jul 20, 2026 at 10:27:29AM +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) > > > > --- > > > > arch/x86/mm/pat/set_memory.c | 21 ++++++++++++--------- > > > > 1 file changed, 12 insertions(+), 9 deletions(-) > > > > > > > > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c > > > > index 301fb9e77d91..a67ca33b9dd1 100644 > > > > --- a/arch/x86/mm/pat/set_memory.c > > > > +++ b/arch/x86/mm/pat/set_memory.c > > > > @@ -439,7 +439,11 @@ static void __cpa_collapse_large_pages(struct cpa_data *cpa) > > > > > > > > list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) { > > > > list_del(&ptdesc->pt_list); > > > > - pagetable_free(ptdesc); > > > > + > > > > + if (folio_test_pgtable(ptdesc_folio(ptdesc))) > > > > + pagetable_dtor_free(ptdesc); > > > > + else > > > > + pagetable_free(ptdesc); > > > > > > Lets not introduce more folio-ptdesc crossovers, we're trying to get > > > rid of them :) > > > > > > I believe pagetable_dtor_free() should do what you're looking for on its > > > own anyway. > > > > Actually, looking at it closer, maybe not because of the conditional > > portion? But that makes me think it might be better to just replace the > > ptdesc_clear_kernel() with pagetable_dtor() in the free function... > > Well some kernel page tables are still allocated without ctor (early allocated > direct map for isntance), and if you did pagetable_dtor_free() it > unconditionally calls pagetable_dtor(). > > The ptlock_free() and __folio_clear_pgtable() there would be harmelss (no locks > assigned for kernel page table, and if PG_table never set clearing it is a noop) > but the lruvec_stat_sub_folio() would cause an unbalanced decrement of > nr_page_table_pages. Gotcha, thanks for the explanation :) > It sucks, but until everything is updated to call the ctor we have to do it this > way :>) Yeah that makes sense. Although I'd rather see the condition as: if(PageTable(ptdesc_page(...))) We really shouldn't be calling ptdesc_folio() anywhere anymore.