From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (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 5879422AE65 for ; Tue, 23 Sep 2025 17:20:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758648043; cv=none; b=uKo9VLyJZvsxoARns9Irv2CJ25sTKpXZpWDGf2TQNBcl1V5ZJP02LbNghmh/hDriIUIwyMMHen3UWstHlpOEofiMrY9wNGPKwRsssBkXB6SDAvKojcpETzRerLEQUaBFmgUIg0R8Ldo0FHx97pEXSxOAZS3ZsUXVges53tnFkkQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758648043; c=relaxed/simple; bh=GQOiGAHKcRUkXEHaDuaQQXU6E9L7x8uBzg9+vx6eMX0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mV09ldlOxecmXDucE82ZGLsnb39Tc41yoWV5JbLWJ7lVwm3LCH5vwC1huV7ML1cVeOui7Vb5aJQ0x+WFhJLU7DdzJf2bOhXBOq8wUPuDAfOS+Q1CeC3MoMQRwOf2Nn6errXHQmUIbX0bwAO09NfYFe/Xfcy8YymY8OSeJqlzhO4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=vCnmUEcO; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="vCnmUEcO" Message-ID: <1a8352a6-e03f-4f3e-a06b-fab757e39eab@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758648028; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zhWRSUVB8GF7kCQH4EGUYZkhuU402qmBEblBNgpnv18=; b=vCnmUEcOJcdG5I4H4EeCK1M+LLbB60ngG817jo83Vo6bdMnlfLHaKSuVzIxH0lInW54Sda Kb/hdqVsJnw0cUz93IyL5x7VjXDNCPhZtPTgyDowkkT90DFJN9QxpeBKp4DZvz0YwPUtEL MgZYJV8EAvGz9D3+TvAY57J8A4ijaoM= Date: Wed, 24 Sep 2025 01:20:16 +0800 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages To: David Hildenbrand , Catalin Marinas Cc: 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 References: <20250922021458.68123-1-lance.yang@linux.dev> <8bf8302a-6aba-4f7e-8356-a933bcf9e4a1@redhat.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <8bf8302a-6aba-4f7e-8356-a933bcf9e4a1@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/9/23 20:00, David Hildenbrand wrote: > On 23.09.25 13:52, Catalin Marinas wrote: >> 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. > > I assume any tag changes would result in CoW. Right, I did a test and confirmed that any attempt to change tags on the shared zero page results in a Copy-on-Write, as expected ;) > > It would be interesting to know if there are use cases with VMs or other > workloads where that could be beneficial with KSM. > >> 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; > > That looks reasonable to me. > > @Lance as you had a test setup, could you give this a try as well with > KSM shared zeropage deduplication enabled whether it now works as > expected as well? This works as expected. Both KSM (with use_zero_pages enabled) and the THP shrinker are now able to successfully merge zero-filled pages with the shared zero page, as long as those pages are not mapped with PROT_MTE. > > Then, this should likely be an independent fix. @Catalin could you send a real patch?