From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 07ABC391E7E for ; Thu, 12 Mar 2026 19:58:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773345514; cv=none; b=cGMXFkh44BlpdkwGLn8il8b3SMzKP2wFWpRVOyF3KCziTzD86dOfvwcbUrAa0mgqtlqa8A9MJxCo3FZ0DeWLeSjOi3k03Jc9n/ANgnSOI5QkAry+aaAyw/vUvdDd5ZuenUkZJntdzhjkKMp8L7RXitu3mEWlkoe5hzvHxqULqow= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773345514; c=relaxed/simple; bh=2oAIBAbeD5lh0AmLKC7kBXJJ4zqW2tjxgJK2/n54DlM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=N+rYgH3utpg8QxmLDz+mlifT6447sNPK2vbMn0lTiFlyKPUuyAbYA0FpivfwPm3twJDXOYG5BHCPXofAHG8GUtbhn7A+3GXXpK/+tZEhANdkIV2QvNzb9jxKVhK6cC7DTJX6mRSReHBona2ycPjadJ+WwWMwpuebXTNmVZb44yU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fJn00l88; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fJn00l88" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DDF02C19425; Thu, 12 Mar 2026 19:58:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773345513; bh=2oAIBAbeD5lh0AmLKC7kBXJJ4zqW2tjxgJK2/n54DlM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=fJn00l883ztTj0W6a4qKGBTVC4OuFVFH0jACQVcA/Ph1OECYYIL40jPNoxXYaSx+6 BoLGgJkVwTuw3r6m6XwSUBu4wXDID2H4TyJ8EBPplSRMPvdtR+rEzcLkuuerqdFqbV ZpV1+P4TKMLt6Ro8eYpYBV2abAfYDuI5wMjU2H9dlCT9yR7fmj7XJ83nDGO2S4yjB/ 87Zj81Psth2YSETQbC9odXcHDk9VMD+o77BMl/dlGFmB0V31dDl7Vz1kwhd9XleeCI DTfhEZAZvqaXK+IUAXo5KdFASU5skkR1+Tz5S9BIuii+YfkygptYY2GEp5G2/RpJsx QKbug7dh6Ii/A== Message-ID: <76c5576a-65d2-4dde-a969-ef6577855864@kernel.org> Date: Thu, 12 Mar 2026 20:58:28 +0100 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 v5 0/6] Migrate on fault for device pages To: mpenttil@redhat.com, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Jason Gunthorpe , Leon Romanovsky , Alistair Popple , Balbir Singh , Zi Yan , Matthew Brost References: <20260211081301.2940672-1-mpenttil@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: 8bit On 2/11/26 09:12, mpenttil@redhat.com wrote: > From: Mika Penttilä > > Currently, the way device page faulting and migration works > is not optimal, if you want to do both fault handling and > migration at once. > > Being able to migrate not present pages (or pages mapped with incorrect > permissions, eg. COW) to the GPU requires doing either of the > following sequences: > > 1. hmm_range_fault() - fault in non-present pages with correct permissions, etc. > 2. migrate_vma_*() - migrate the pages > > Or: > > 1. migrate_vma_*() - migrate present pages > 2. If non-present pages detected by migrate_vma_*(): > a) call hmm_range_fault() to fault pages in > b) call migrate_vma_*() again to migrate now present pages > > The problem with the first sequence is that you always have to do two > page walks even when most of the time the pages are present or zero page > mappings so the common case takes a performance hit. > > The second sequence is better for the common case, but far worse if > pages aren't present because now you have to walk the page tables three > times (once to find the page is not present, once so hmm_range_fault() > can find a non-present page to fault in and once again to setup the > migration). It is also tricky to code correctly. One page table walk > could costs over 1000 cpu cycles on X86-64, which is a significant hit. > > We should be able to walk the page table once, faulting > pages in as required and replacing them with migration entries if > requested. > > Add a new flag to HMM APIs, HMM_PFN_REQ_MIGRATE, > which tells to prepare for migration also during fault handling. > Also, for the migrate_vma_setup() call paths, a flag, MIGRATE_VMA_FAULT, > is added to tell to add fault handling to migrate. > > One extra benefit of migrating with hmm_range_fault() path > is the migrate_vma.vma gets populated, so no need to > retrieve that separataly. > > Tested in X86-64 VM with HMM test device, passing the selftests. > For performance, the migrate throughput tests from the selftests > show similar numbers (within error margin) as unmodified kernel. > Tested also rebased on the > "Remove device private pages from physical address space" series: > https://lore.kernel.org/linux-mm/20260130111050.53670-1-jniethe@nvidia.com/ > plus a small patch to adjust with no problems. Stumbling over this in my backlog, so far we have to RB tags. I'm going to take a look at the core MM bits (soon I hope), but it would be great if other people could review the HMM bits and provide proper tags. -- Cheers, David