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 32A4738D3E1; Fri, 22 May 2026 21:47:41 +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=1779486463; cv=none; b=NG5mYy0Of1QyI82x7ud5v7X2MNwhL8kIM/3VoNrDasrWy+fpqmis045gvz3isKbpwn+f0rfHT1/wnfyPCI5f8jPvUMeiMf2/wNsN9lFwaAN8FIDh1xpzzXvdxAmz9lDPaIO02wWntvzkoBl37B1SaZBNh9FFMCJCKMzL1VZJKns= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779486463; c=relaxed/simple; bh=J5I2v1pkkrtnLqIA0bctNNUJE7BR8yuMo+VxJwgNymE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GR48Hz9ZOnl00KRwT9CLs/JznHFngq0xgMpgElquDwruaeY/Yt5tHLb9AleML1VW1RfUY2FutdfiS9q8HB+xirfb4CMC8RQqm0JOppmzuvOyBgT3vsj1ig+P/saefpCOseNwnF05CstAxEGO0bxVJsQfxvBUUKg9OOwy7+XmZOg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NQFXKOfk; 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="NQFXKOfk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BB9531F000E9; Fri, 22 May 2026 21:47:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779486461; bh=dljCxK1YsQZK68Gu9mqCcSDyHCzwiebiXLJPqO1ehDg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NQFXKOfkWjqTi+NS7Vynjpq9Z7Lngi3QzlylGeaZ114+OMZfavNUzkIR0G8inmBDj H346fIolGEZCV1B55hPO1XYWFoMPyr+ruDlBgXWbbLzjfQ2IcJ37hfsrobCXH9PFb0 9waRpIVjasjyzEWSomq2KOW9ChUnvNXE9f4AVq5tatndZPBYQOe9GUxYegSrfH8qo/ VBJOsNgnAcN2Cfg1GeZifTa6EUHLC+2C7MUKh5RQQ7/VwrYbYbRwvfE+IpkdCtWu0m C/nM7SpktkbhFg8Wvj9wBKi4O+YBlFkT1sMu7IW8ohEVECKUVP2yBn2HAmTJrarZ2B 3Iicltj3psmcQ== Message-ID: Date: Fri, 22 May 2026 23:47:25 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v18 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, Usama Arif References: <20260522150009.121603-1-npache@redhat.com> <20260522150009.121603-7-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 5/22/26 17:00, Nico Pache wrote: > Pass an order and offset 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, and offset indicates were in the PMD to > start the collapse attempt. > > 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. > > Acked-by: Usama Arif > Signed-off-by: Nico Pache > --- I guess we should add a comment here like: /* * Only notify about the PTE range we will actually modify. While we * temporary unmap the whole PTE table for mTHP collapse, we'll remap * it later, leaving other PTEs effectively unmodified. The locks we hold * prevent anybody from stumbling over such temporarily unmapped PTE tables. */ > > - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address, > - address + HPAGE_PMD_SIZE); > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, start_addr, > + end_addr); > mmu_notifier_invalidate_range_start(&range); > > pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */ > @@ -1294,26 +1297,23 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a > * Parallel GUP-fast is fine since GUP-fast will back off when > * it detects PMD is changed. > */ > - _pmd = pmdp_collapse_flush(vma, address, pmd); > + _pmd = pmdp_collapse_flush(vma, pmd_addr, pmd); > spin_unlock(pmd_ptl); > mmu_notifier_invalidate_range_end(&range); > tlb_remove_table_sync_one(); > > - pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); > + pte = pte_offset_map_lock(mm, &_pmd, start_addr, &pte_ptl); > if (pte) { > - result = __collapse_huge_page_isolate(vma, address, pte, cc, > - HPAGE_PMD_ORDER, > - &compound_pagelist); > + result = __collapse_huge_page_isolate(vma, start_addr, pte, cc, > + order, &compound_pagelist); > spin_unlock(pte_ptl); > } else { > result = SCAN_NO_PTE_TABLE; > } > > if (unlikely(result != SCAN_SUCCEED)) { > - if (pte) > - pte_unmap(pte); > spin_lock(pmd_ptl); > - BUG_ON(!pmd_none(*pmd)); > + WARN_ON_ONCE(!pmd_none(*pmd)); Likely VM_WARN_ON_ONCE is sufficient. > /* > * We can only use set_pmd_at when establishing > * hugepmds and never for establishing regular pmds that > @@ -1321,21 +1321,24 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a > */ > pmd_populate(mm, pmd, pmd_pgtable(_pmd)); > spin_unlock(pmd_ptl); > - anon_vma_unlock_write(vma->anon_vma); > goto out_up_write; > } > > /* > - * All pages are isolated and locked so anon_vma rmap > - * can't run anymore. > + * For PMD collapse all pages are isolated and locked so anon_vma > + * rmap can't run anymore. For mTHP collapse the PMD entry has been > + * removed and not all pages are isolated and locked, so we must hold > + * the lock to prevent neighboring folios from attempting to access > + * this PMD until its reinstalled. > */ That makes sense. I was wondering whether there was another reason for dropping the anon_vma lock ... I guess it was just for latency purposes given that there was no actual need for the lock anymore once all folios in the range were isolate+locked. With the two its above addressed Acked-by: David Hildenbrand (arm) -- Cheers, David