From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 529C93563FA; Mon, 1 Jun 2026 08:15:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780301757; cv=none; b=D2oQmbxMCdr0kZwk/vWSi0LNAaSopgDRyPYd7ELYArmSE85klZc5Zo3GIqSQbY2Tba+yeSs0DIRtrcSDGyZzXo20uIe5y/++FPvNqC68H0xIpOzd7sMQ/0yWxzBz9G4OPNL5T8xSB5i3whdAkxNxbf7SCFqpEsH4ZJf5C3/Kn8Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780301757; c=relaxed/simple; bh=mWi7WwsgTW6pSJiwpj1EaR5L0KHT6josjO6ni5jUtYE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QtpbnHwP/XjMHwtHPoIU44fRcQsTvurIqOAYY/kp2W9nqo6S9xRzPhVv1ujNAuLgFzcU6cgWcl98nFueBxh8YjNmf2vFkScxj9t/rpAGMHwGJ+3S5i6Ys7gNIytDOPM2R/bg4MoAP2/ToGUN6mlov9f1QGg3saPDmthlmgxtkQs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UXqYnVYy; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="UXqYnVYy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78A701F00893; Mon, 1 Jun 2026 08:15:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780301756; bh=7Fe6JYGyHSbXJX3O8KSfKT2Vqw40RbPEy7q1vSIqR20=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=UXqYnVYyrTNj3AbXP5FO8iA3F+UJFpvf8kNk4cUVukmp3d14TGCjR24dISkoXzY00 Nk13Ahn5ewSSi2UDtW7sIE/fczQu5QcGTaBCfZ09UmsaY7oKSRO4o8tGVZyftEt35h JXOGf+iSgGPslC2095u/u9ChwJsEaPCP2cfP/sTkreHxILwL2IXJ33cPrzhHquvvv/ yRrGLIWJXN+3Mu6es5RWGfKWLFRGsoBOa70nea1qIMPCf12OzqsDDL6A0TZ77lwbNA /dvgFUyAop3rVmVWwWohUBBudV4qdXIboac50dBV4llgHHSimEgh5CRgO+uEEHK31b wS2KS6+JZZHwQ== Message-ID: <1c99294c-9ebe-4856-bfde-09801701d75c@kernel.org> Date: Mon, 1 Jun 2026 10:15:38 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse To: Lance Yang 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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit 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. -- Cheers, David