From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (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 D97F73876B7 for ; Mon, 1 Jun 2026 07:51:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780300298; cv=none; b=XHI5Vm4Istw252zoEbMA66jzI8dmtQmDXCoBA8JF15FqgDrZhWdvG/cZ7hZMskqaP5omEIw/dP36BaAK6ilRKFXNeEUuSzbK6vYYnl1djjN8fU2J3YVGWsFCTxPhpdumo41EjAgwN4/t69qSl0XxE10Ecof6dHlTwxUxfv79Oso= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780300298; c=relaxed/simple; bh=LhlJZ0Ot242vtrJ/74zhndDm9aMQIgd9VwLmmV4oPHM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=foqJBAMmNyGoDqJxLQ3QofRGJgto5oQzr41CPsflRQMaAOE3TcxLqhgOiywxf4vEbUUCr5LpE2LLzpx2Bd5KMm/WAElH+cxz/eJy2J/IBGLwqWEuBFpp6hLkZ5N0MF5lW/5zSE76Wg8dANkVQQMKVg+09AOIQlqaAxUvRBvCaZU= 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=I6w2WUk6; arc=none smtp.client-ip=95.215.58.171 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="I6w2WUk6" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780300283; 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=xdixgpShH7hYpW9sMn1MMnWna0aWkb3QEt/4zti9SVk=; b=I6w2WUk6rUegOHmHPsCN+sU/WijwIVnKCXVcyzIteQdgNXRTPeOOhpcSHT3+10mY+wLtzg IAi0m7kp4H10MnJHt9y8RR7DiqInQQLPDeCRMbGQDyzwk3oSoJRXK+GNveolJHCSPX5qIP 4XbQrnx3pi0RV1OcITXmtcmXWh9Ez1M= Date: Mon, 1 Jun 2026 15:49:42 +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> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2026/6/1 14:54, David Hildenbrand (Arm) wrote: > On 6/1/26 05:28, Lance Yang wrote: >> >> On Sun, May 31, 2026 at 10:00:17PM +0200, David Hildenbrand (Arm) wrote: >>> On 5/31/26 11:39, Lance Yang wrote: >>>> >>>> >>>> Emm ... is it safe to use map_anon_folio_pte_nopf() here? >>>> >>>> At this point pmdp_collapse_flush() has cleared the PMD from the page >>>> tables. The PTE table we are updating is only reachable through the saved >>>> old PMD value, _pmd, until pmd_populate() below. >>>> >>>> map_anon_folio_pte_nopf() does set_ptes() and then calls >>>> update_mmu_cache_range(). Documentation/core-api/cachetlb.rst describes >>>> that hook as: >>>> >>>> " >>>> At the end of every page fault, this routine is invoked to tell >>>> the architecture specific code that translations now exists >>>> in the software page tables for address space "vma->vm_mm" >>>> at virtual address "address" for "nr" consecutive pages. >>>> " >>>> >>>> But that does not seem true here yet, since the PTE table is not >>>> reachable from vma->vm_mm when update_mmu_cache_range() is called. >>>> >>>> Should we avoid calling update_mmu_cache_range() until after the PTE >>>> table is reinstalled with pmd_populate()? >>> >>> I recall that update_mmu_cache* users mostly care about updating folios flags, >>> for the folio derived from the PTE ... or flushing caches for the user address. >>> >>> So intuitively I would say "the architecture code doesn't care that the PMD >>> table will only be visible to HW shortly after". The important thing should be >>> that it will definetly happen, and that nothing else is curently there or can be >>> there? >> >> 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 ... Cheers, Lance >> >> 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() */ > + if (pte_ptl != pmd_ptl) > + spin_lock(pte_ptl); > pmd_populate(mm, pmd, pmd_pgtable(_pmd)); > + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false); > + if (pte_ptl != pmd_ptl) > + spin_unlock(pte_ptl); > } > spin_unlock(pmd_ptl); > > >