From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-187.mta1.migadu.com (out-187.mta1.migadu.com [95.215.58.187]) (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 589891F875A for ; Fri, 19 Sep 2025 02:45:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.187 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758249926; cv=none; b=IlOL6KfzpTkNh9kvbiy3hO2pUAGZKk8ho3cEA6YEjIA/aSkEl+JL4v3fiyBr2kH59SiMOH9wsiKkdgbdi3xAKNwhAQpQyC63O2ONFUHyze2rW7gGnLnmIBPUIVri7+SQaRaLgoF+1D0Z+W0DKnYfzGiH7H7crmCpXvkuA+NS2Po= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758249926; c=relaxed/simple; bh=6zFDx2Zuzm7i23PZ+F83cFt4yBUktJbixiLg31FQI5k=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JkxTFska/4m7Qh681jkzdcjtjIxyk6mxittFKpqhBJ6JrlfW8QmWNwJUcPvTRG3eKna+i/ujxauv0oeyILYsoHMrqPoTYkWo6IhdL+WWy6h0kUEwfax3UxVL3OtUkNBJDvhSlQuQrXTiIPh/e5lTcI1upIlO7VmlONX7Z4gKUJE= 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=kkZnUCqS; arc=none smtp.client-ip=95.215.58.187 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="kkZnUCqS" Message-ID: <20e0fb73-3476-4b1c-959e-c1af38f8e1a0@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758249922; 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=pEd4CRxp0Qy3iytKkYSUgyoNAPH5uPPdhfTDAiqFt+Q=; b=kkZnUCqSVKS2D7eOJzEGuV3v+zGK9iIe7bTiuC/CxNG5peUXCkMg8QwHhrIUCzAA3fPGtj H5dS3iVmdASVXo6qgjNsHY2nKaNPMk5MXf8dk4yu9aiRvB+36+Nq6NC4YvSuED6HU0WwFj /DE5F2zJxguD24b6ukaUVsKcEQQp55k= Date: Fri, 19 Sep 2025 10:44:26 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs Content-Language: en-US To: Zi Yan Cc: akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@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> <6BAE5498-D0AB-40F5-85F8-B92A05CA72AD@nvidia.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <6BAE5498-D0AB-40F5-85F8-B92A05CA72AD@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2025/9/19 03:12, Zi Yan wrote: > On 18 Sep 2025, at 1:04, 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 >> --- >> mm/khugepaged.c | 10 ++++++++++ >> 1 file changed, 10 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 9ed1af2b5c38..70ebfc7c1f3e 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, >> result = SCAN_PTE_UFFD_WP; >> goto out_unmap; >> } >> + /* >> + * Guard PTE markers are installed by >> + * MADV_GUARD_INSTALL. Any collapse path must >> + * not touch them, so abort the scan immediately >> + * if one is found. >> + */ >> + if (is_guard_pte_marker(pteval)) { > > pteval is already is_swap_pte(), would is_guard_swp_entry() be a better > choice here? Save one is_swap_pte() call. Yeah. Good spot! Will do ;) Thanks, Lance > >> + result = SCAN_PTE_NON_PRESENT; >> + goto out_unmap; >> + } >> continue; >> } else { >> result = SCAN_EXCEED_SWAP_PTE; >> -- >> 2.49.0 > > > Best Regards, > Yan, Zi