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 CBCA742123A for ; Wed, 12 Aug 2026 11:41:17 +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=1786534880; cv=none; b=tUDqe/uiuCnw8lwgkZEnobb0m8tLpFdunVtlOju3U0tfloyRyM0dm1YVeIoQXbIrbl3+tx5RBT+jbdz7jF+9Cri99c4o9eyx236Fz/tVfyAHLKBP7eVVX/UyAOIUcuVG08RQrYgI6u1vS2D9E1AfTsk2YACEC3AedceB93zypOc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786534880; c=relaxed/simple; bh=j4h3A3DU7j9SBucCz6BZ0AEh5ez8mYjFiuvW3qrVz3w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=LM8jnhneaNpTS6tiAuNbPK2lRi2XgW0O/w9jt6kCIuHN4gZEMnn3pi4H+QikQpzx5rsgHqC7OkdgNBjUjZ/KJ/ctfCRWIx1ykt8dWTdtYlVKcH5VN6VceFaJQ/VhU48xfcsUv1FBWWO0TpcjBL3LgwHLHVl84DBL06Z+gAKKzeY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hFNsG5v7; 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="hFNsG5v7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 615B71F000E9; Wed, 12 Aug 2026 11:41:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786534877; bh=H3gZ1FMK6CoTy8pVEciCEE7NzrDYyMSkC6mU77m6EGc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hFNsG5v7l1bYLH8AHxJxTQurUxLaGlnnjZizxaZlF/UEhI5QdtOB5BAiCbSRo/Mfj not+fjdMpz3CVNigeJdaM2uQryJnkDJD8NFLC5ihwXF2pEi5Me6go11Pbv7z/I+wvE 1bFE2d6Exx9+k4kqXSGQ7JtpX9M5soPFZb2lmqp9+qiOyzctZynL536Fdv2faUIEiR vBec35/Y2WYYh0VEMnUpdwAWtn+O+Xe7LXH/O34Bb6onANgav5zs/AequvluT/4sCc fGSYgVAqyNPCFgONkao4zDVJlU1s36Wsz6NU9lN6o1cZ84v+38L9Fa4dEcg8MkRalV 5hsIWsZ7EmhnQ== Message-ID: <5d91760e-79e6-4bb7-9c41-52d8ab17ee82@kernel.org> Date: Wed, 12 Aug 2026 13:41:13 +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: [RFC PATCH v3 0/8] batch lookups in follow_page_mask() To: Rik van Riel , linux-kernel@vger.kernel.org Cc: kernel-team@meta.com, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , linux-mm@kvack.org References: <20260811025157.1632867-1-riel@surriel.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 8/11/26 04:51, Rik van Riel wrote: > follow_page_mask() walks the page tables one page at a time, even when the > caller asked for a whole run of contiguous pages. Every page of a large folio > re-enters the pmd/pud/pte walk and re-takes the page table lock. > > This series changes follow_page_mask() to return a page count and a fill an > array of pages, instead of a single struct page, so a walker can hand back > more than one page per call. > > Patches 1 to 4 are preparation, no functional change: > > 1: move __get_user_pages()'s open-coded pages[] fill and cache flush into a > gup_fill_pages() helper, which the rest of the series reuses. > 2: convert the follow_page_mask()/follow_p4d_mask()/follow_pud_mask()/ > follow_pmd_mask()/follow_page_pte() call chain to return a long instead of > a struct page pointer or ERR_PTR(). Every path still handles one page. > 3: split the "commit to a resolved page" tail of follow_page_pte() into > follow_page_pte_commit(). > 4: split the "work out which page this PTE maps" half of follow_page_pte() > into follow_one_pte(), leaving one unlock and one exit. > > Patch 5 has the huge page paths store the page and leave the array fill to > follow_pud_mask()/follow_pmd_mask() after they unlock, so the cache flushes > happen outside the pud/pmd critical section. > > Patch 6 has follow_huge_pud()/follow_huge_pmd() report the huge page's real > subpage count instead of a separate *page_mask output, and retires *page_mask > and __get_user_pages()'s try_grab_folio() call and subpage loop. > > Patch 7 walks every PTE in a page table in one follow_page_pte() call instead > of one per page. > > Patch 8 adds follow_pte_batch() so a contiguous same-folio run is committed > with one refcount grab. This is the only patch whose benefit depends on folio > size; patch 7 alone covers plain base pages. > > Patches 7 and 8 carry their own benchmark tables, both measured against the > base of the series, so the split between the two mechanisms is visible: the > single-call walk is worth 2.3x on base pages and 2.2x on 64 kB mTHP, and > refcount batching adds a further 5.9x on the mTHP case. > > v3: > - split up the series into 8 much smaller patches (David & Lorenzo) > - shorten changelogs where they were too long (Lorenzo) > - fix FOLL_WRITE folio dirtying by gathering dirty bits from all PTEs I'll try getting to this soon. As I raised previously, the whole follow_page_* terminology is just stale, and likely we should just not add new functions that use this terminology. I.e., follow_page_pte_commit() -- Cheers, David