From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta0.migadu.com (out-180.mta0.migadu.com [91.218.175.180]) (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 0BD0F389118 for ; Mon, 1 Jun 2026 08:45:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780303518; cv=none; b=sMKOx+xjpLL1Nei9k1H5go2tIGnejFBC1jPxi/9vQlHTs1aZd2A3wd9XY9jdu2697zWym8ld54xSbRT4MHg3gkrkI1Zax1Oy/0bLOyuMdvyQQKhRwYvycuoCLAZz6GJygY6hysSNBndbiXDJKiARdSE3TyXrXIl0d4LeIJYZj5g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780303518; c=relaxed/simple; bh=GTUzk1xbOR3HrMGPr1vzXbCMH8PmLMq7Js6eGVC2H6I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JtKHkDr/G+4z+FyAqq4Hyu085rg7JvcnRl6KA9OzkNJ8palBiLcQ0AYXHoQgZillNkz4QDqIGaED0CsCn4fqdE1bkcW2D+HrkR9olOivyD6XX4RFX6WJwqR1t38yqnUQXp3FvyLgjsMwhPW0g+sUi2w2JUItfMek6dR8aZF0ro4= 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=ZLZ7xn9h; arc=none smtp.client-ip=91.218.175.180 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="ZLZ7xn9h" Message-ID: <32751424-8ad3-4fd2-9f07-8a4f5a98d632@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780303510; 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=KOza5WLjJJP8CiSXeBVxPUywkC8JLKE6qF7U7SNImWM=; b=ZLZ7xn9hizllj61bp6haxPz5lrd2xU6GM6bDFO7OqfBI8FyYU42nZlvDfUN0Z5UMU7aoPF 43AKsI66i32+vOBgOtACggTlFYEWqqH+kqMMNwAHNssdinbwxhK69jz7fMZGpnr4PB5lUn G1D4AFqa1xpKp9BarKxWXLQdyayfjMk= Date: Mon, 1 Jun 2026 16:44:44 +0800 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse Content-Language: en-US To: "David Hildenbrand (Arm)" Cc: npache@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com, usama.arif@linux.dev References: <2024af56-5e99-4799-a586-e9ba756cecb9@kernel.org> <20260601032804.96122-1-lance.yang@linux.dev> <1c99294c-9ebe-4856-bfde-09801701d75c@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <1c99294c-9ebe-4856-bfde-09801701d75c@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2026/6/1 16:15, David Hildenbrand (Arm) wrote: > On 6/1/26 09:49, Lance Yang wrote: >> >> >> On 2026/6/1 14:54, David Hildenbrand (Arm) wrote: >>> On 6/1/26 05:28, Lance Yang wrote: >>>> >>>> >>>> Ah, fair point. >>>> >>>> I was mostly worried about arch hooks that walk vma->vm_mm again, rather >>>> than only using the pte pointer passed in. For example, mips does: >>> >>> Right, a re-walk would be the real problem. >>> >>>> >>>>    update_mmu_cache_range() >>>>      -> __update_tlb() >>>>        -> pgd_offset(vma->vm_mm, address) >>>>        -> pte_offset_map(...) >>>> >>>> and __update_tlb() has this assumption: >>>> >>>>         /* >>>>          * update_mmu_cache() is called between pte_offset_map_lock() >>>>          * and pte_unmap_unlock(), so we can assume that ptep is not >>>>          * NULL here: and what should be done below if it were NULL? >>>>          */ >>>> >>>> So if khugepaged happens to run with current->active_mm == vma->vm_mm >>>> here, could __update_tlb() hit the none PMD, get NULL from >>>> pte_offset_map(), and then dereference it? >>> >>> Likely yes -- that MIPS code is horrible. And the comment in MIPS code >>> even spells that out. :( >>> >>> Do you know about other code like that, or is MIPS the only one doing a >>> re-walk and crossing fingers? >> >> I had Codex do the boring grep-work through the arch update_mmu_cache* >> code :D >> >> MIPS doesn't seem to be the only code doing a re-walk, but it is the >> only one I found that appears to assume the PMD/PTE walk cannot fail, >> without checking whether the PMD is none ... > > Okay, but likely the other code that tries to handle it is also problematic. > > Best to make sure the page table is already installed when updating the entries. Neat, makes sense to me :D That way the page talbe is back in place before any arch hook gets to look at it :)