From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 D7B472E8881; Tue, 23 Sep 2025 11:52:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758628335; cv=none; b=OwNWhH7jSya6i+TaXUb4uIKu9xpCdOp5+HQzP50vI8tYlBrn9dxXlKyH+qokJsT/UHvEJ6tJYTVsDchoWgcH1k4eE5LRhS/8hCjT/0ulEb11M5QC8ykrzsWzZJCGCTmUPRlCqoFkgv0jzhtR6Zp9rqIDiDtYlXiZkNkIhonqw5g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758628335; c=relaxed/simple; bh=EsovoJk3iO97HWqD4/DkC87+I2irN6cySa4mX0rbxFY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NpcFeCm7rrbogPO7UYmGYi6BJZXxM+oH54F4kRZuCl0MuuFYaQXmG3UXrXNGq57NAdq8i+v4CDLFzL40OM+ZMuF1ZWzBmgxN8caqf36IM88S3FBVLJye9ZcADuj9rMAu8XCgE1YLAf0dgcbgGxXrlj6zS8mQPhEneRaoDNkyIXk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2910BC4CEF5; Tue, 23 Sep 2025 11:52:09 +0000 (UTC) Date: Tue, 23 Sep 2025 12:52:06 +0100 From: Catalin Marinas To: David Hildenbrand Cc: Lance Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, usamaarif642@gmail.com, yuzhao@google.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, voidice@gmail.com, Liam.Howlett@oracle.com, cerasuolodomenico@gmail.com, hannes@cmpxchg.org, kaleshsingh@google.com, npache@redhat.com, riel@surriel.com, roman.gushchin@linux.dev, rppt@kernel.org, ryan.roberts@arm.com, dev.jain@arm.com, ryncsn@gmail.com, shakeel.butt@linux.dev, surenb@google.com, hughd@google.com, willy@infradead.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, qun-wei.lin@mediatek.com, Andrew.Yang@mediatek.com, casper.li@mediatek.com, chinwen.chang@mediatek.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages Message-ID: References: <20250922021458.68123-1-lance.yang@linux.dev> 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 Mon, Sep 22, 2025 at 07:59:00PM +0200, David Hildenbrand wrote: > On 22.09.25 19:24, Catalin Marinas wrote: > > On Mon, Sep 22, 2025 at 10:14:58AM +0800, Lance Yang wrote: > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > index 32e0ec2dde36..28d4b02a1aa5 100644 > > > --- a/mm/huge_memory.c > > > +++ b/mm/huge_memory.c > > > @@ -4104,29 +4104,20 @@ static unsigned long deferred_split_count(struct shrinker *shrink, > > > static bool thp_underused(struct folio *folio) > > > { > > > int num_zero_pages = 0, num_filled_pages = 0; > > > - void *kaddr; > > > int i; > > > for (i = 0; i < folio_nr_pages(folio); i++) { > > > - kaddr = kmap_local_folio(folio, i * PAGE_SIZE); > > > - if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { > > > - num_zero_pages++; > > > - if (num_zero_pages > khugepaged_max_ptes_none) { > > > - kunmap_local(kaddr); > > > + if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) { > > > + if (++num_zero_pages > khugepaged_max_ptes_none) > > > return true; > > > > I wonder what the overhead of doing a memcmp() vs memchr_inv() is. The > > former will need to read from two places. If it's noticeable, it would > > affect architectures that don't have an MTE equivalent. > > > > Alternatively we could introduce something like folio_has_metadata() > > which on arm64 simply checks PG_mte_tagged. > > We discussed something similar in the other thread (I suggested > page_is_mergable()). I'd prefer to use pages_identical() for now, so we have > the same logic here and in ksm code. > > (this patch here almost looks like a cleanup :) ) > > If this becomes a problem, what we could do is in pages_identical() would be > simply doing the memchr_inv() in case is_zero_pfn(). KSM might benefit from > that as well when merging with the shared zeropage through > try_to_merge_with_zero_page(). Yes, we can always optimise it later. I just realised that on arm64 with MTE we won't get any merging with the zero page even if the user page isn't mapped with PROT_MTE. In cpu_enable_mte() we zero the tags in the zero page and set PG_mte_tagged. The reason is that we want to use the zero page with PROT_MTE mappings (until tag setting causes CoW). Hmm, the arm64 memcmp_pages() messed up KSM merging with the zero page even before this patch. The MTE tag setting evolved a bit over time with some locking using PG_* flags to avoid a set_pte_at() race trying to initialise the tags on the same page. We also moved the swap restoring to arch_swap_restore() rather than the set_pte_at() path. So it is safe now to merge with the zero page if the other page isn't tagged. A subsequent set_pte_at() attempting to clear the tags would notice that the zero page is already tagged. We could go a step further and add tag comparison (I had some code around) but I think the quick fix is to just not treat the zero page as tagged. Not fully tested yet: diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c index e5e773844889..72a1dfc54659 100644 --- a/arch/arm64/kernel/mte.c +++ b/arch/arm64/kernel/mte.c @@ -73,6 +73,8 @@ int memcmp_pages(struct page *page1, struct page *page2) { char *addr1, *addr2; int ret; + bool page1_tagged = page_mte_tagged(page1) && !is_zero_page(page1); + bool page2_tagged = page_mte_tagged(page2) && !is_zero_page(page2); addr1 = page_address(page1); addr2 = page_address(page2); @@ -83,11 +85,10 @@ int memcmp_pages(struct page *page1, struct page *page2) /* * If the page content is identical but at least one of the pages is - * tagged, return non-zero to avoid KSM merging. If only one of the - * pages is tagged, __set_ptes() may zero or change the tags of the - * other page via mte_sync_tags(). + * tagged, return non-zero to avoid KSM merging. Ignore the zero page + * since it is always tagged with the tags cleared. */ - if (page_mte_tagged(page1) || page_mte_tagged(page2)) + if (page1_tagged || page2_tagged) return addr1 != addr2; return ret; -- Catalin