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 64B0E37E2FB for ; Tue, 2 Jun 2026 16:35:06 +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=1780418110; cv=none; b=LkRHKHbyZY4KdT0pAU7K2uhBolwiq3KP2rr+I7WJhTB/qtZL3zW8fUFZS92t69ApCtp5yRfBnh+061i/3SgFJzALdHx0kBY5lCHGWb75vYkrQ+MR7NQDlRhe5YJmuxD5jp5p8/3yFc6fqTpMaESLPa7ol/3UVUf3qVRBQzBEmSw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780418110; c=relaxed/simple; bh=e02MIFR4Eh5QfCCofm7+5HoQnYnpURbtiujJHVzOfS8=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=thmPAeaVpIUMVnwZcUrgAAlbdKeiBlIqzwKlev+uTD8DYp1ohgufTuY3qm4cff9W/2DuQV/Se981WsTGY5R0P938t8eHCZR872N+BY/EN2prYa961Ql9zMHbW+Af/oIk0iH4Vo6NJFv7BrzCvOS7yzrn0QG6HFcM35xgC8wsS5w= 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=e99alI/o; 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="e99alI/o" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780418093; 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=Kfv65sDfPDmPDpEO0E3tjxWYqpXB1G5lz0Quc0XLk54=; b=e99alI/oamPJv8ECRvV4dpEn6AtfCTtrQLYfTTiheBxLC0V7F4073yDgp+SJ8btK+1Bls+ 489j1GGe0TmK1/VotWwJHV1ZBblOz+AnoFF6aywKWaSeg1yk/EUaSnEX8fmav4ytNOvWDs 35D+wl50XsMRVVFSKGbug9oWaVNSKEI= From: Lance Yang To: npache@redhat.com Cc: lance.yang@linux.dev, david@kernel.org, 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 Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse Date: Wed, 3 Jun 2026 00:34:18 +0800 Message-Id: <20260602163418.93877-1-lance.yang@linux.dev> In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Tue, Jun 02, 2026 at 09:30:06AM -0600, Nico Pache wrote: >On Mon, Jun 1, 2026 at 4:48 AM Lance Yang wrote: >> >> >> >> 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! > >are we sure? that folio_mark_uptodate is done before the PTEs are >reinstalled. Then we reinstall the PMD right after. Currently >separated by the smp_wmb(). Reinstalling the PMD first makes the PTE table reachable again, right? So before pmd_populate(), we only need to order the old PTE clears before the PTE table is reachable again; __folio_mark_uptodate() already has the smp_wmb() for that :) The new PTEs are filled later under the PTL. Hopefully I didn't miss soemthing :) >I was copying this from other THP code that performs similar PTE/PMD juggling. > >I can remove it, but I'd rather air on the side of caution with this. Cheers, Lance