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 E6FF6C7EE23 for ; Wed, 1 Mar 2023 05:42:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229518AbjCAFmc (ORCPT ); Wed, 1 Mar 2023 00:42:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32878 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229480AbjCAFmb (ORCPT ); Wed, 1 Mar 2023 00:42:31 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1E27D30EBC; Tue, 28 Feb 2023 21:42:30 -0800 (PST) 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=v2VBdt3kHKVqHXSdicN5o2cil7i5B6UknsXaVF5uIhU=; b=DywQDkHUM4ppZeg43fbbwjP2M2 /oZO+6xgBiP8hwhJbgWiTP69FQRD4/TRYreUdNCf3f3ItX6aQ6ooZyRGSKDJtl2hi5PQQX4aIklSg kkxhaosMgcf/f6Edwdrw1DDqkninZ/HzXgIXFhn/IsHGBU/TBcbzb2xHGu9Ge9dvTSbOdP41aX/Sh NIew+5py+V0Pp6iglRuW2MOGodiC7wk4PmF/ZPgOsEUOKjaPdnWOi84FNtU1EKrNdgUVc3JCRjj5s /u6r9TIoXEQj3dEuOpitvrS7BGqiK8TJuB/+BKvrrHw8PmEb8VEyY72snQpJ4SwUWiQcPA/Z8Hxe1 m4zgbPmg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pXFEK-001Ph8-Mb; Wed, 01 Mar 2023 05:42:20 +0000 Date: Wed, 1 Mar 2023 05:42:20 +0000 From: Matthew Wilcox To: Gao Xiang Cc: Theodore Ts'o , lsf-pc@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org Subject: Re: [LSF/MM/BPF TOPIC] Cloud storage optimizations Message-ID: References: <49b6d3de-e5c7-73fc-fa43-5c068426619b@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <49b6d3de-e5c7-73fc-fa43-5c068426619b@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Mar 01, 2023 at 01:09:34PM +0800, Gao Xiang wrote: > On 2023/3/1 13:01, Matthew Wilcox wrote: > > On Wed, Mar 01, 2023 at 12:49:10PM +0800, Gao Xiang wrote: > > > > The only problem is that the readahead code doesn't tell the filesystem > > > > whether the request is sync or async. This should be a simple matter > > > > of adding a new 'bool async' to the readahead_control and then setting > > > > REQ_RAHEAD based on that, rather than on whether the request came in > > > > through readahead() or read_folio() (eg see mpage_readahead()). > > > > > > Great! In addition to that, just (somewhat) off topic, if we have a > > > "bool async" now, I think it will immediately have some users (such as > > > EROFS), since we'd like to do post-processing (such as decompression) > > > immediately in the same context with sync readahead (due to missing > > > pages) and leave it to another kworker for async readahead (I think > > > it's almost same for decryption and verification). > > > > > > So "bool async" is quite useful on my side if it could be possible > > > passed to fs side. I'd like to raise my hands to have it. > > > > That's a really interesting use-case; thanks for bringing it up. > > > > Ideally, we'd have the waiting task do the > > decompression/decryption/verification for proper accounting of CPU. > > Unfortunately, if the folio isn't uptodate, the task doesn't even hold > > a reference to the folio while it waits, so there's no way to wake the > > task and let it know that it has work to do. At least not at the moment > > ... let me think about that a bit (and if you see a way to do it, feel > > free to propose it) > > Honestly, I'd like to take the folio lock until all post-processing is > done and make it uptodate and unlock so that only we need is to pass > locked-folios requests to kworkers for async way or sync handling in > the original context. > > If we unlocked these folios in advance without uptodate, which means > we have to lock it again (which could have more lock contention) and > need to have a way to trace I/Oed but not post-processed stuff in > addition to no I/Oed stuff. Right, look at how it's handled right now ... sys_read() ends up in filemap_get_pages() which (assuming no folio in cache) calls page_cache_sync_readahead(). That creates locked, !uptodate folios and asks the filesystem to fill them. Unless that completes incredibly quickly, filemap_get_pages() ends up in filemap_update_page() which calls folio_put_wait_locked(). If the filesystem BIO completion routine could identify if there was a task waiting and select one of them, it could wake up the waiter and pass it a description of what work it needed to do (with the folio still locked), rather than do the postprocessing itself and unlock the folio. But that all seems _very_ hard to do with 100% reliability. Note the comment in folio_wait_bit_common() which points out that the waiters bit may be set even when there are no waiters. The wake_up code doesn't seem to support this kind of thing (all waiters are non-exclusive, but only wake up one of them).