From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 C6C6A1B7F7 for ; Tue, 25 Jun 2024 14:07:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719324425; cv=none; b=MA8iA4Qd9UFziJBTQguPKFqJKuRBTu2f6x78NLBqK53KaD3T06fuBpYg9dltbwWH3WD+YQS27y5J8YzwwYYZR309Y1zDTn2rtRHaFT9Tqair56OMsuJxqTsIUeHViE71TWU5YM3pdC0+H0SL3LH4kTa1Gm3sVWwxbYoaOWhVxs8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719324425; c=relaxed/simple; bh=HurV4YlLhMcDPGGBVRrfcSvNLYL6IdEFP1wBB8tq47w=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=OZZp7EF18KNLB2yOqSw3LHY31CYKbVAWiMWQsWJs0o4iMLlHn3l2rcbsIiOe03Yae61FQCHbP5WeZTxLAEZbRmwi0K3uHNb+FptFUrq5ezHa0tiNvFNUUGUkj+32v6duO0d5R38Sm26yiCiwZY98Bjt6GBFksRqSj5+uifbJQ70= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=b3anXdf6; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="b3anXdf6" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=hGiPM6C5L/nRqsdu3jq0C7CbnwzvLeffETFeFxTX0+E=; b=b3anXdf6y1JVACdaTmYa87Qdwz IXn7N2bJIfzk9KNwWXeg8tumh7lqXZTBdx3GEINGv9BNJyMrvBKM/tmlyIK+vjWWO7lMBqI2/gW4N XQP8c9u0eYpqkmTL11wjuLbSi2rQCqK2/qoiw2rq7DlGfa9/ApwsOpqEbj3j0JZ+Uy58IC72wpm44 RVUjYWXi/WdkaINNa8n4X8Kgwq91tskrXsMl9afIlOrW59G3xNzWOCh56uVJUJram+C2/i0JjGNRT VvpU11sKddLUsxQfVklfujUBPT4Ny+My7k0dM+8H17DBjtzWVsKSKnjwEbtWus2udI+8mjTe8+C63 EkRPsf+A==; Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1sM6oT-0000000BCMT-2TZf; Tue, 25 Jun 2024 14:06:25 +0000 Date: Tue, 25 Jun 2024 15:06:25 +0100 From: Matthew Wilcox To: Ryan Roberts Cc: Baolin Wang , Kefeng Wang , Catalin Marinas , Will Deacon , Ard Biesheuvel , Marc Zyngier , James Morse , Andrey Ryabinin , Andrew Morton , Mark Rutland , David Hildenbrand , John Hubbard , Zi Yan , Barry Song <21cnbao@gmail.com>, Alistair Popple , Yang Shi , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "Yin, Fengwei" , linux-arm-kernel@lists.infradead.org, x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6 18/18] arm64/mm: Automatically fold contpte mappings Message-ID: References: <20240215103205.2607016-19-ryan.roberts@arm.com> <1285eb59-fcc3-4db8-9dd9-e7c4d82b1be0@huawei.com> <8d57ed0d-fdd0-4fc6-b9f1-a6ac11ce93ce@arm.com> <018b5e83-789e-480f-82c8-a64515cdd14a@huawei.com> <43a5986a-52ea-4090-9333-90af137a4735@linux.alibaba.com> <306874fe-9bc1-4dec-a856-0125e4541971@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Jun 25, 2024 at 02:41:18PM +0100, Ryan Roberts wrote: > On 25/06/2024 14:06, Matthew Wilcox wrote: > > On Tue, Jun 25, 2024 at 01:41:02PM +0100, Ryan Roberts wrote: > >> On 25/06/2024 13:37, Baolin Wang wrote: > >> > >> [...] > >> > >>>>> For other filesystems, like ext4, I did not found the logic to determin what > >>>>> size of folio to allocate in writable mmap() path > >>>> > >>>> Yes I'd be keen to understand this to. When I was doing contpte, page cache > >>>> would only allocate large folios for readahead. So that's why I wouldn't have > >>> > >>> You mean non-large folios, right? > >> > >> No I mean that at the time I wrote contpte, the policy was to allocate an > >> order-0 folio for any writes that missed in the page cache, and allocate large > >> folios only when doing readahead from storage into page cache. The test that is > >> regressing is doing writes. > > > > mmap() faults also use readahead. > > > > filemap_fault(): > > > > folio = filemap_get_folio(mapping, index); > > if (likely(!IS_ERR(folio))) { > > if (!(vmf->flags & FAULT_FLAG_TRIED)) > > fpin = do_async_mmap_readahead(vmf, folio); > > which does: > > if (folio_test_readahead(folio)) { > > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > > page_cache_async_ra(&ractl, folio, ra->ra_pages); > > > > which has been there in one form or another since 2007 (3ea89ee86a82). > > OK sounds like I'm probably misremembering something I read on LWN... You're > saying that its been the case for a while that if we take a write fault for a > portion of a file, then we will still end up taking the readahead path and > allocating a large folio (filesystem permitting)? Does that apply in the case > where the file has never been touched but only ftruncate'd, as is happening in > this test? There is obviously no need for IO in that case, but have we always > taken a path where a large folio may be allocated for it? I thought that bit was > newer for some reason. The pagecache doesn't know whether the file contains data or holes. It allocates folios and then invites the filesystem to fill them; the filesystem checks its data structures and then either issues reads if there's data on media or calls memset if the records indicate there's a hole. Whether it chooses to allocate large folios or not is going to depend on the access pattern; a sequential write pattern will use large folios and a random write pattern won't. Now, I've oversimplified things a bit by talking about filemap_fault. Before we call filemap_fault, we call filemap_map_pages() which looks for any suitable folios in the page cache between start and end, and maps those.