From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 466DCA55 for ; Thu, 18 Sep 2025 11:12:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758193925; cv=none; b=OhDB1wlNIoc3APvQC0qb/aWs+rRwCdMNxpxCyFEejzjBOYpdOx5O02D87R06YMtioCA5M9S9NKWqPDZtyBAzQZ5MMzt/rAB/WRpAWXJaDV88LPCv2yqi7qmhFznDkJSD1kNjx1igt9sYgmd4qR7C/nSe9RuTEJi6NPwCltYsosE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758193925; c=relaxed/simple; bh=FLEsT8JAr3pGNNcKahnr/nd1z8+z2fgK7EWbnj5Coyk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=TTgtoAtzzL9cM/DWfjxOhv38nhoy6nEKxq4cdztT/BOv1OwI9ewhNqDhAo6C5EYuMJfOqv/yyeSDNiQt/Z0bApBGAlSc5SaWli1GcWldfbbvP/ai9Hs6Zq3bLiNddEPcq+pfsgo5dfVn1VdxJgdkXSzCVB/1FiPYkFm2Yh/ttCU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 77B711762; Thu, 18 Sep 2025 04:11:55 -0700 (PDT) Received: from [10.164.18.52] (MacBook-Pro.blr.arm.com [10.164.18.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1AA663F66E; Thu, 18 Sep 2025 04:11:58 -0700 (PDT) Message-ID: <8f49bd75-5314-43e4-aecc-e0ef3616f7fd@arm.com> Date: Thu, 18 Sep 2025 16:41:56 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs To: Lance Yang , akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, ioworker0@gmail.com, kirill@shutemov.name, hughd@google.com, mpenttil@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250918050431.36855-1-lance.yang@linux.dev> <20250918050431.36855-3-lance.yang@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: <20250918050431.36855-3-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 18/09/25 10:34 am, Lance Yang wrote: > From: Lance Yang > > Guard PTE markers are installed via MADV_GUARD_INSTALL to create > lightweight guard regions. > > Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when > encountering such a range. > > MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in > the special marker in __collapse_huge_page_swapin(). > > hpage_collapse_scan_pmd() > `- collapse_huge_page() > `- __collapse_huge_page_swapin() -> fails! > > khugepaged's behavior is slightly different due to its max_ptes_swap limit > (default 64). It won't fail as deep, but it will still needlessly scan up > to 64 swap entries before bailing out. > > IMHO, we can and should detect this much earlier. > > This patch adds a check directly inside the PTE scan loop. If a guard > marker is found, the scan is aborted immediately with SCAN_PTE_NON_PRESENT, > avoiding wasted work. > > Suggested-by: Lorenzo Stoakes > Signed-off-by: Lance Yang > --- Reviewed-by: Dev Jain