From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D10AF480978; Tue, 21 Jul 2026 12:09:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784635788; cv=none; b=qOcPkXQk5bE/X4guDdmit5GiuStGPfMpXe9j7Py+Tla03hCOGXNTzChDtFdS+FWbc+AFe5advzWH+eVbeR5V82NSJPC4l47y/RvLysVCLKWy0PPIlv1qrvkMaygaT1BnLOUjJqubMKG4HKvBhU4qu2BeYSHRRuns7phqvPxizdY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784635788; c=relaxed/simple; bh=A7RpNfh6HhENIkWk/z+a/o2A0wzRVZZLzyn0aLn7sRg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=CxFp+G8G5j5c5h4By7i6M87LbmI1VRXroZEYJl7U7BCXDB8EDkFDk8i4agLH5DGU4agoa6U+1Bt3jzM8tKQ8wb4c1AtCtfQzxFOYgFnR+gFZsOIlTJW4+A1hDsqeb9ca8d8xTC+OaMQVox098FFipNHgRt/jBR4c3QL+LPU0NuU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FCBchnQk; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="FCBchnQk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 328B31F000E9; Tue, 21 Jul 2026 12:09:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784635786; bh=+aU74TjwGAKlM1ykbFleIgZ0kqOSlolWT3M16XgHiR4=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=FCBchnQktpU1hJI6VVEm3hDfoYdq9iEkrTct/AwFrdSPMeIM2JM9lcfnYGZvRazXy a8/zEgTWzzu21EtZuE/yXLKIsr8+xmCSw8VQto9H55e/F7BOkcmdPrGKpFlnLdU6re P5k4ZG9I+odTrG5rVGkxSNyesNhUmgrLt1GhgnhPRojWuMUoJrqjXdLb7V3XMqg5AD TVSfO+pgPO/2Q2pQFDfTf/qYvPC89XKyTy1TvbKgp8UuJBe1ZqU0Pds2kZC0rvCNPJ uFFsxHyaTsWjBb2HyzjjQXmE1nd7xxIcOhjwTfRrF7994SGksKawr0zsLyM2nHMz5D hsUSrDxoKoW0g== Date: Tue, 21 Jul 2026 13:09:27 +0100 From: "Lorenzo Stoakes (ARM)" To: Mike Rapoport Cc: Vishal Moola , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , 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: iommu@lists.linux.dev 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 01:00:53PM +0100, Lorenzo Stoakes (ARM) wrote: > On Tue, Jul 21, 2026 at 01:32:44PM +0300, Mike Rapoport wrote: > > On Tue, Jul 21, 2026 at 10:58:50AM +0100, Lorenzo Stoakes (ARM) wrote: > > > On Tue, Jul 21, 2026 at 02:45:43AM -0700, Vishal Moola wrote: > > > > > > > > > > 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(). > > > > TBH, I cannot think of a scenario when page tables allocated at boot would > > be collapsed. But surely, checking the page type is safer just in case. > > Yeah nor can to be honest, anything that could be made large in the direct map > would already be large right? > > But it's 'just in case' somebody did something dumb :) Later can maybe make it a > WARN_ON(). But just to fix the proximate issue for now. > > > > > > > > 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 :) > > > > > > No worries, this is subtle stuff with lots of weird gotchas and stuff we need to > > > improve... I seem to have fallen down an unexpected rabbit hole with these fixes > > > :) > > > > > > > > > > > > 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. > > > > > > I think better for a follow up since the code already uses ptdesc all over the > > > place (fundamental to the approach really, keeping a list of page tables etc.) > > > and this is a fix that needs backporting. > > > > I agree with Vishal that it's better to use page type rather than folio > > type. And it's the same for backporting ;-) > > Ah sorry misunderstood, you mean straight up PageTable(ptdesc_page()), I thought > Vishal was saying we shouldn't be directly referencing ptdesc's at all (which > would be the rework). > > I guess definitionally page tables are never folios. I lazily went with what I > saw elsewhere, my bad :) Ah yeah I remember now, i saw __pagetable_ctor() dealt with folios: static inline void __pagetable_ctor(struct ptdesc *ptdesc) { struct folio *folio = ptdesc_folio(ptdesc); __folio_set_pgtable(folio); lruvec_stat_add_folio(folio, NR_PAGETABLE); } And was like 'huh?' (surely definitionally they're _not_ folios) but went with that on that basis. Another place to clean up I guess? (that one _definitely_ is a follow up though ;) > > Will respin! > > > > > > > Cheers, Lorenzo > > > > -- > > Sincerely yours, > > Mike. > > Cheers, Lorenzo Cheers, Lorenzo