From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2FCF5C433EF for ; Thu, 2 Jun 2022 16:47:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237218AbiFBQr1 (ORCPT ); Thu, 2 Jun 2022 12:47:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56694 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237256AbiFBQrO (ORCPT ); Thu, 2 Jun 2022 12:47:14 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8354A183A4; Thu, 2 Jun 2022 09:47:12 -0700 (PDT) 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=uif769frfrYihV6YiWxMGs5eBDpigCO/7kp1qxb3WTw=; b=X1J8cjMICzgCsxkFRDuSKDibuq +hwtjZyiS864Fr7r7uq4aQq4PFJK8VVPtTvA3dgtknuPKyw4EBX1JB9s+nB3KGFIgggx0e3+GLv3u vbMu5+YaAl+ytycERctp3YK+2C3G7+LSwiYpjGTDu3vDFtLiYXFNWI6jC5/2Q830MFIX++tvDHvXZ YvlqbcBvFT8uBesnZGkUqv2HHz+vBTH52qY0pR6Zv4ulCVylu2shMJjutQoPXpL42ScCwY04aQYHA 3aDk5gG94UFrHq7cR6qV4Oihdo5S6BMUAH89Wa7kSar0mhBXuMzt/ZEiI+pTDTLkoWTa+hq31TEtr btMsSDLw==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1nwnyC-007HMt-Qg; Thu, 02 Jun 2022 16:46:48 +0000 Date: Thu, 2 Jun 2022 17:46:48 +0100 From: Matthew Wilcox To: Ira Weiny Cc: David Sterba , "Fabio M. De Francesco" , Chris Mason , Josef Bacik , David Sterba , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/3] btrfs: Replace kmap() with kmap_local_page() Message-ID: References: <20220531145335.13954-1-fmdefrancesco@gmail.com> <20220601132545.GM20633@twin.jikos.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Thu, Jun 02, 2022 at 09:22:15AM -0700, Ira Weiny wrote: > On Wed, Jun 01, 2022 at 03:25:45PM +0200, David Sterba wrote: > > On Tue, May 31, 2022 at 04:53:32PM +0200, Fabio M. De Francesco wrote: > > > This is the first series of patches aimed towards the conversion of Btrfs > > > filesystem from the use of kmap() to kmap_local_page(). > > > > We've already had patches converting kmaps and you're changing the last > > ones, so this is could be the last series, with two exceptions. > > > > 1) You've changed lzo.c and zlib. but the same kmap/kunmap pattern is > > used in zstd.c. > > I checked out zstd.c and one of the issues there is the way that the input > workspace is mapped page by page while iterating those pages. > > This got me thinking about what Willy said at LSFmm concerning something like > kmap_local_range(). Mapping more than 1 page at a time could save some > unmap/remap of output pages required for kmap_local_page() ordering. Umm ... Not entirely sure what I said, but it'd be really hard to kmap multiple pages with the current PAE implementation. I've steered away from doing that for now, and kmap_local_folio() just guarantees the page that the offset lands in is mapped. I don't think the right answer is having a kmap_folio() that will map the entire folio. I'd be more tempted to add vmap_folio() for that. My understanding is that PAE systems have more address space available for vmap than they do for kmap. > Unfortunately, I think the length of the input is probably to long in many > cases. And some remapping may still be required. > > Cc: Willy > > As an aside, Willy, I'm thinking that a kmap_local_range() should return a > folio in some way. Would you agree? I imagine it taking a folio to describe the range that's being accessed. But maybe it should be a phys_addr_t? I tend to prefer phys_addr_t over pfn + offset as it's more compact on 64-bit systems and the same on 32-bit systems.