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 2320F362130; Fri, 5 Jun 2026 17:48:32 +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=1780681720; cv=none; b=Oi/oCrStVSQsh53xKm3NFqdNiwzsRRtGlqLv/mUC4nIdgUyOx5W3CZy/jzk2paro5d1oH0WEZvy6LsXkrQR0W5qhfZt1kbGvghHrTNlXQZHvo2Lmm0tMKSS9PB8RfIKfDJF3yIS6Dj7xoRVCTU2xj+/mviJDmG+7xNcvj88wehs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780681720; c=relaxed/simple; bh=ZFDtEeT144MWLEjdbF0WkH/WzViFG0lLgkOssxKLH2Y=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=FxGCse+EOtgs1BAWqXiqkL2iqSPwhOcb39GNyrzEziru6gnzd2Bt0YXqKpQoZ8h6foiyuOHYIWG9aevz298KuQBQpnYFCsCuiJU88A2vfeKyHsDbV3QVTsToxZ2cRal8aM1KJSRqQmyM/IDm4W6JJ0IS1J+nxMf8SiHKugJQwoA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jkmcYYzG; 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="jkmcYYzG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 42FAB1F00893; Fri, 5 Jun 2026 17:48:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780681712; bh=YmJ+4Vv5SanG71jFELjTxWcwVOj/fu6/r0NzV9M26yY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=jkmcYYzGWD1lltC9KBboZ1Lk+Z7lzUvhC0L38QUnjDAgpIVdLzxM4rSkeHUSC56I6 LRqIgNAuJdT7yOMQbeuVCpvRJR7AcMx55vCnYU42EoM7ZO9XKIUTqf/kY7/WjVL4eV +QyLpQbEPC53OPS7IbCMrtTe6Y3Q3eS5fbLP1h5gY993zsb5QSHBSFf15i1o6YVdiN JunHCvLbLkpNMZ7xU4hkShwaf39/fWu+GHRiI0r07GrwEi/mE7Gj8bUFy61aILA3iG 0SYuJTFXPafclmUBLLVVFvrRMfeGsxoiEVXfcBqGOUVfOFf1CHLE0+M3Q8sd6DkaoV SiLucUWeZQrIA== Message-ID: <95390529-3a80-473c-9433-958db7a2dc6c@kernel.org> Date: Fri, 5 Jun 2026 19:48:17 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v19 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse To: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: 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, lance.yang@linux.dev, 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 References: <20260605161422.213817-1-npache@redhat.com> <20260605161422.213817-7-npache@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/5/26 18:14, Nico Pache wrote: > Pass an order to collapse_huge_page to support collapsing anon memory to > arbitrary orders within a PMD. order indicates what mTHP size we are > attempting to collapse to. > > For non-PMD collapse we must leave the anon VMA write locked until after > we collapse the mTHP-- in the PMD case all the pages are isolated, but in > the mTHP case this is not true, and we must keep the lock to prevent > access/changes to the page tables. This can happen if the rmap walkers hit > a pmd_none while the PMD entry is currently unavailable due to being > temporarily removed during the collapse phase. > > To properly establish the page table hierarchy without violating any > expectations from certain architectures (e.g. MIPS), we must make sure to > have the PMD reinstalled before the PTEs, and hold both PTE/PMD locks > before calling update_mmu_cache_range() (if they are distinct locks). > > Signed-off-by: Nico Pache > --- [...] > */ > __folio_mark_uptodate(folio); > - pgtable = pmd_pgtable(_pmd); > - > spin_lock(pmd_ptl); > - BUG_ON(!pmd_none(*pmd)); > - pgtable_trans_huge_deposit(mm, pmd, pgtable); > - map_anon_folio_pmd_nopf(folio, pmd, vma, address); > + VM_WARN_ON_ONCE(!pmd_none(*pmd)); > + if (is_pmd_order(order)) { > + pgtable = pmd_pgtable(_pmd); > + pgtable_trans_huge_deposit(mm, pmd, pgtable); > + map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); > + } else { > + /* > + * Some architectures (e.g. MIPS) walk the live page table in > + * their implementation. update_mmu_cache_range() must be called > + * with a valid page table hierarchy and the PTE lock held. > + * Acquire it nested inside pmd_ptl when they are distinct locks. > + */ > + if (pte_ptl != pmd_ptl) > + spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING); > + 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); > > folio = NULL; > > result = SCAN_SUCCEED; > out_up_write: > + if (anon_vma_locked) > + anon_vma_unlock_write(vma->anon_vma); > + if (pte) > + pte_unmap(pte); We re-enable some page table walkers before we unmap the PTE. We still hold the mmap lock in write mode, so nothing would currently try reclaiming the page table concurrently. So I guess this works right now, but we should likely rework that code later to either revert both statements. Or maybe we can simply unmap like we did, and simply remap before we call map_anon_folio_pte_nopf()? Remapping should not fail. Alternatively to an unmap+remap, I think we could also unmap earlier for PMD diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 6de935e76ceb..ba2a2508dda6 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1378,6 +1378,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s if (is_pmd_order(order)) { anon_vma_unlock_write(vma->anon_vma); anon_vma_locked = false; + pte_unmap(pte); + pte = NULL; } result = __collapse_huge_page_copy(pte, folio, pmd, _pmd, But this can also be handled later. We now hold an anon_vma lock a bit longer for !pmd-collapse. But there is also less to copy. If that bites us, we can try optimizing later. So after another skim, I think this patch is ready for primetime. We can address the things mentioned above later ... and any fallout can be fixed later, if any. Acked-by: David Hildenbrand (Arm) -- Cheers, David