From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta0.migadu.com (out-183.mta0.migadu.com [91.218.175.183]) (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 ACE41384235 for ; Mon, 1 Jun 2026 09:08:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780304931; cv=none; b=BXzXSs2CZiKDQNJ7cqnCV6TtkhY51qC0rqg61XjgekHnPK3YJ+CYKOdZfR/xLssOqM2som0BdgnO4XgJGXknxoOZn0q0XvN2Y0PK3F9xEaTKjUfvw7fbDSqP+VZ7mxr9uEV+qvWTzCn9lDpxJbtJB2V/e/7rnaKqnJabEiBimbE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780304931; c=relaxed/simple; bh=92tT17PKhvQMu6q/YGqPwoLQeT6L41Yel4oIjhnlZSM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=scPzGVRy9i3Tcah8cACosXnLoEmM+y4WyruveBtbmTKS/Jt2tqlFylrbP0vKfTktEbyoL+IX6O+WaiqzG+HP1tcELGEMxlcCWQAofKwNzyeVcedIRtpeVVfgugx/uIj0yT7nxA5rB59ltzvQjaTDAcpH+MfF9SGJ3j5ZWkJ8FIk= 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=KxtyW1yS; arc=none smtp.client-ip=91.218.175.183 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="KxtyW1yS" Message-ID: <616de1a8-1cfd-40b8-b04f-7b324be40bfd@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780304917; 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=VK++rQga/i6Qi+LoraofDjTFK/zBvGqjLp0spI1XrxM=; b=KxtyW1yS8Bmvb5wfbc9LV85WFpETDo57OXPUYHvytoo6p29K7gINb2qVNlWklQtImBbsCy BZSH9y6GovK0MeDLyz+cyKp1wb+IItvlaXIN9a9BkWzX99qxM2xoSFd0wS3MFukyIO5Zaf MJkTSeNaWw9rlrbF0NUQyagIy3Fpuq8= Date: Mon, 1 Jun 2026 17:08:11 +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? > >> >> 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? > + 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); > Cheers, Lance