From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) (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 BE2C8391853 for ; Mon, 1 Jun 2026 10:48:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780310903; cv=none; b=ee2ddYcEccoYBswJpTZR4R5gnB8r5cp+h+CA8KhFV+g04lw2Vu2HuK7iVIOVztCEWd1YJ/4TTxWM5ubzVd8RzYmIhAnNWnGJxbsXh1X1B4zZz3OLJo9sEvRy0Gp5JgpOJmJP5InViwuCq3gBPxQO0JwNZg2EbPjSgFYqbY5EvlE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780310903; c=relaxed/simple; bh=T7HEqqKt/hcYTLG5nwxbWXpj7+Okw4kr4HMN7yBXN3g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Py2lmDoHbcuO1thYseMHcEtbXvo88hBluXqPDigR6Au6eg6p2ULj3qb8X2WOyt+JsO5Wdksq6kW+Xx6livpA2XIQ8lF7E57buD/bHVnS7/Obb1WLiL/VfNNkNcIdYcWf4X4qTxfu/s5TQIGc8Yknv0FiaKDYJJmw0ZCB3HCOmR8= 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=neaGU8o1; arc=none smtp.client-ip=91.218.175.178 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="neaGU8o1" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780310899; 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=CpP3p+OZ5kTTKtnC7lknJ8SUiTBUDhdsLvtMvZmEm/4=; b=neaGU8o1iE3/4nTUTm1ddWoYsE2dfW3eX7wU62CONcntJ79HSV+NHfJSc9y6ThIun4wasJ BP4QDwazz2Xy7sFW7AuQhOUSjEUvavOzPE0+8qTglgU72VGjV5ffSvP3JWtwytKfZnfQwP 88YxT/UrKu+ag27omxnIeveYLz889ok= Date: Mon, 1 Jun 2026 18:47:46 +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> <616de1a8-1cfd-40b8-b04f-7b324be40bfd@linux.dev> <6b11bf0a-769c-4ef2-ac6f-2af38200a6bc@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <6b11bf0a-769c-4ef2-ac6f-2af38200a6bc@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2026/6/1 18:23, David Hildenbrand (Arm) wrote: > On 6/1/26 11:08, 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? >>> >>>> >>>> Just wanted to raise it since some arch code may still have assumptions >>>> like this, and the always-enable-mTHP work is getting closer ... >>> >>> Right. I assume set_pte_at() couldn't trigger something similar (re-walk) in >>> arch code, >>> because we simply provide the ptep. update_mmu_cache_range() only consumes the >>> pte. >>> >>>> >>>> Probably very very very hard to hit, though :) >>> >>> Delaying update_mmu_cache_range() is nasty, as we'd have to make sure that >>> nobody can interfere in the meantime ... and the PMD lock will not be sufficient. >>> >>> Maybe we could reinstall the page table with the cleared (none) entries while >>> still holding the PTL? >>> >>> Thinking out loud: >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 5ba298d420b7..e39b750b1e6f 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1413,13 +1413,17 @@ static enum scan_result collapse_huge_page(struct >>> mm_struct *mm, unsigned long s >>>                  map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); >>>          } else { >>>                  /* >>> -                * set_ptes is called in map_anon_folio_pte_nopf with the >>> -                * pmd_ptl lock still held; this is safe as the PMD is expected >>> -                * to be none. The pmd entry is then repopulated below. >>> +                * Re-insert the page table with the cleared entries, but >>> +                * hold the PTL, such that no one can mess with the re-installed >>> +                * page table until we updated the temporarily-cleared entries >>> +                * through map_anon_folio_pte_nopf(). >>>                   */ >>> -               map_anon_folio_pte_nopf(folio, pte, vma, start_addr, / >>> *uffd_wp=*/ false); >>> -               smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */ >> >> One small thing, I think we should probably keep the smp_wmb(), and just >> move it before the earlier pmd_populate(). >> >> IIUC, the ordering we want is still: >> >>   clear old PTEs >>   smp_wmb() >>   pmd_populate() >> >> so another CPU cannot walk through the re-installed PMD and still observe >> the old PTEs, right? > > There is a smp_wmb() in __folio_mark_uptodate(), that should be sufficient? Ah, cool! __folio_mark_uptodate() already does the job :P So yeah, no extra smp_wmb() needed here! Cheers, Lance