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 7D6A236D9FE for ; Fri, 2 Oct 2026 18:53:14 +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=1790967195; cv=none; b=azAgNP/lQlzP2OahC04nt8tMlWL+lrWmYYmGeTk3VkwE5cdV2V/76FS9ovw16W2WN1T+goH/ffYYzCtyeY1kydyNkiaPvJWBWo9C6H3J9Eo4Jp+enz9uMbNLgHQmc2p2hcJRY9SnXL6kZQlh/Wxqq+srJyRSg0EeUmH9L6cREc8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790967195; c=relaxed/simple; bh=LaLiCiB4+sl20X0NaWrtQO3X2OtmtfrqtaJGKu1lz6U=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QuoxYduFo4a2zJcRyp7LtJKfx7efvAHGNcEcL8TiSmJXgGqwx9eMaIx1DTIwF5YVqeqDqu87mVCcZMzFB4xn/nnNZqHUQfQcFH/U2LnzGOD3CV4YzAPco14FH+kkC0xRhr54JU2aQEFcmA/aZolJIR/PVKVBooHy8rDc5r7utxI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HGzUzC6f; 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="HGzUzC6f" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2F061F000FF; Fri, 2 Oct 2026 18:53:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790967194; bh=3zheeU3EuzyWIQHVUugGMtNSaN9Adh+jr5Zkc1USMfk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HGzUzC6f4aroci2M61+q5sGquB80q1nj1gcRlPhIu3eax9X+7c56OCAu+6wYNlABB 7faeIbFwpTgogflebupr6NZIIoQ6Ed5xcHLe9KnguCADZngZw5didjCfxOfHhGTkwc A/tCJvLgMnxirehIyE8R713SJyX7TDlQi+ORzqyzmI/GaidGiPI0Vj6KVBlgp74uzO nRW0ZurqANeYHJFHSzfUUtiFbOZgWLRQhAePeMTghm9LA+XjKQwfwkMSvvbJk66cvg IeH7HBQfVbJk+a+ZYDld9E6T7GdnygpLZGGjC86A57vELVeTEYw8hqrJ9zNuFiVW8+ ejLoBmkUWQlXg== Message-ID: <580fbfd5-2db7-4c46-9335-003ac99d1db9@kernel.org> Date: Fri, 2 Oct 2026 20:53:07 +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 v2 1/2] mm/migrate: walk runs of consecutive pages in do_pages_stat_array() To: Qiliang Yuan , Andrew Morton , Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261002-bug-mm-move-pages-stat-batch-v2-0-f73b5d20519f@gmail.com> <20261002-bug-mm-move-pages-stat-batch-v2-1-f73b5d20519f@gmail.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 10/2/26 03:25, Qiliang Yuan wrote: > move_pages() with a NULL node list reports the node of each page. RDMA > and KV-cache transfer engines use it to find where large registered > buffers live, querying every 4K page of buffers that span hundreds of > gigabytes. > > do_pages_stat_array() looks up the VMA and walks the page tables from > the top for every address, taking and dropping the PTE lock each time. > That costs about 105 ns per page, so a 16 GiB buffer takes 486 ms. > > Callers almost always pass consecutive addresses. Group them into runs > and walk each run with walk_page_range(), which looks up each VMA and > PTE table once and answers every page under it while holding the lock. > Report pages as folio_walk_start() with FW_ZEROPAGE found them: the node > of a normal folio, -EFAULT for the zero page or an address outside any > VMA, and -ENOENT otherwise. Handle PUD and hugetlb leaves in their own > callbacks so that the walk never splits them. > > On 7.3-rc5 in a 16-vCPU VM, querying every page of a populated 4 GiB > buffer: > > before after > 4K pages 105 ns 31.9 ns > THP 90 ns 23.1 ns > > Suggested-by: Zi Yan > Signed-off-by: Qiliang Yuan > --- > mm/migrate.c | 188 ++++++++++++++++++++++++++++++++++++++++++++++++++--------- > 1 file changed, 162 insertions(+), 26 deletions(-) That's a lot of churn ... which is really a shame, because all we want to walk is folio ranges. Oscar was working on a better page table walker API (but I was too busy to provide review so far :( ), which sounds strategically like the better long-term solution. Is there particular need to optimize this in the ns range for 4 KiB of memory immediately? -- Cheers, David