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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 17871C433EF for ; Fri, 1 Apr 2022 18:12:22 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id B8B9A8D0003; Fri, 1 Apr 2022 14:11:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B36BE8D0002; Fri, 1 Apr 2022 14:11:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A00668D0003; Fri, 1 Apr 2022 14:11:50 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0082.hostedemail.com [216.40.44.82]) by kanga.kvack.org (Postfix) with ESMTP id 8EB798D0002 for ; Fri, 1 Apr 2022 14:11:50 -0400 (EDT) Received: from smtpin16.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 495861828D81C for ; Fri, 1 Apr 2022 18:11:40 +0000 (UTC) X-FDA: 79309103160.16.5508263 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf02.hostedemail.com (Postfix) with ESMTP id 1CF628003A for ; Fri, 1 Apr 2022 18:11:38 +0000 (UTC) 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=9ewXUzXXHRK5ASrjbgUPLH+r5wr18GSbGAgNboofTr8=; b=B7/O42F5kP1XBVHtP1ijLUAZvz WGNfBlbIIjNVRpoujdb7T5E+PRUwXt45JPZdaLrn6gY+lLJWbvNP2n45Tdzjcq5fpGQBFnc4NI313 f7/YexjIKWM7zHbvITbt3/7tokxc4aBln74uWV+oa6mu26GG39/odZhmynIL8I7Mwgl9Mcj5yfkzk ADUp9Vo4g0lqTGrDINBS0p5t6IP4F3JCS/G4mvA8w/jRiBSmG47faBEcHBHvLkDm6Wfmj0vVp4j/L KqCkSMkxtvrWYq42stkB2gw4099eiQ3zFHY8i86vT3er7aC5CZb25zbG282OH8/YDKeIDNzAHTN/M 0z0mJS7g==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1naLkC-003SIC-FC; Fri, 01 Apr 2022 18:11:32 +0000 Date: Fri, 1 Apr 2022 19:11:32 +0100 From: Matthew Wilcox To: NeilBrown Cc: Andrew Morton , Jonathan Corbet , Linux Memory Management List , linux-doc@vger.kernel.org Subject: Re: [PATCH] MM: minor improvements to readahead documentation Message-ID: References: <164879346851.25542.18299715584610241983@noble.neil.brown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <164879346851.25542.18299715584610241983@noble.neil.brown.name> X-Stat-Signature: iykt9exh63cdd5t3w9wfddn7fhz8eo33 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 1CF628003A Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b="B7/O42F5"; dmarc=none; spf=none (imf02.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org X-Rspam-User: X-HE-Tag: 1648836698-762420 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Apr 01, 2022 at 05:11:08PM +1100, NeilBrown wrote: > > - use "readahead" consistently - not "read-ahead" or "read ahead". > - clarify some sections that, on reflection, weren't very clear > - minor punctuation/spelling fixes. Coincidentally, I had a number of other changes to this documentation which conflicted throughout with yours. Here's a merge of the two: @@ -13,29 +13,29 @@ * * Readahead is used to read content into the page cache before it is * explicitly requested by the application. Readahead only ever - * attempts to read pages that are not yet in the page cache. If a - * page is present but not up-to-date, readahead will not try to read + * attempts to read folios that are not yet in the page cache. If a + * folio is present but not up-to-date, readahead will not try to read * it. In that case a simple ->readpage() will be requested. * * Readahead is triggered when an application read request (whether a - * systemcall or a page fault) finds that the requested page is not in + * system call or a page fault) finds that the requested folio is not in * the page cache, or that it is in the page cache and has the - * %PG_readahead flag set. This flag indicates that the page was loaded - * as part of a previous read-ahead request and now that it has been - * accessed, it is time for the next read-ahead. + * readahead flag set. This flag indicates that the folio was read + * as part of a previous readahead request and now that it has been + * accessed, it is time for the next readahead. * * Each readahead request is partly synchronous read, and partly async - * read-ahead. This is reflected in the struct file_ra_state which - * contains ->size being to total number of pages, and ->async_size - * which is the number of pages in the async section. The first page in - * this async section will have %PG_readahead set as a trigger for a - * subsequent read ahead. Once a series of sequential reads has been + * readahead. This is reflected in the struct file_ra_state which + * contains ->size being the total number of pages, and ->async_size + * which is the number of pages in the async section. The readahead + * flag will be set on the first folio in this async section to trigger + * a subsequent readahead. Once a series of sequential reads has been * established, there should be no need for a synchronous component and - * all read ahead request will be fully asynchronous. + * all readahead request will be fully asynchronous. * - * When either of the triggers causes a readahead, three numbers need to - * be determined: the start of the region, the size of the region, and - * the size of the async tail. + * When either of the triggers causes a readahead, three numbers need + * to be determined: the start of the region to read, the size of the + * region, and the size of the async tail. * * The start of the region is simply the first page address at or after * the accessed address, which is not currently populated in the page @@ -45,14 +45,14 @@ * was explicitly requested from the determined request size, unless * this would be less than zero - then zero is used. NOTE THIS * CALCULATION IS WRONG WHEN THE START OF THE REGION IS NOT THE ACCESSED - * PAGE. + * PAGE. ALSO THIS CALCULATION IS NOT USED CONSISTENTLY. * * The size of the region is normally determined from the size of the * previous readahead which loaded the preceding pages. This may be * discovered from the struct file_ra_state for simple sequential reads, * or from examining the state of the page cache when multiple * sequential reads are interleaved. Specifically: where the readahead - * was triggered by the %PG_readahead flag, the size of the previous + * was triggered by the readahead flag, the size of the previous * readahead is assumed to be the number of pages from the triggering * page to the start of the new readahead. In these cases, the size of * the previous readahead is scaled, often doubled, for the new @@ -65,52 +65,52 @@ * larger than the current request, and it is not scaled up, unless it * is at the start of file. * - * In general read ahead is accelerated at the start of the file, as + * In general readahead is accelerated at the start of the file, as * reads from there are often sequential. There are other minor - * adjustments to the read ahead size in various special cases and these + * adjustments to the readahead size in various special cases and these * are best discovered by reading the code. * - * The above calculation determines the readahead, to which any requested - * read size may be added. + * The above calculation, based on the previous readahead size, + * determines the size of the readahead, to which any requested read + * size may be added. * * Readahead requests are sent to the filesystem using the ->readahead() * address space operation, for which mpage_readahead() is a canonical * implementation. ->readahead() should normally initiate reads on all - * pages, but may fail to read any or all pages without causing an IO + * folios, but may fail to read any or all folios without causing an I/O * error. The page cache reading code will issue a ->readpage() request - * for any page which ->readahead() does not provided, and only an error + * for any folio which ->readahead() did not read, and only an error * from this will be final. * - * ->readahead() will generally call readahead_page() repeatedly to get - * each page from those prepared for read ahead. It may fail to read a - * page by: + * ->readahead() will generally call readahead_folio() repeatedly to get + * each folio from those prepared for readahead. It may fail to read a + * folio by: * - * * not calling readahead_page() sufficiently many times, effectively - * ignoring some pages, as might be appropriate if the path to + * * not calling readahead_folio() sufficiently many times, effectively + * ignoring some folios, as might be appropriate if the path to * storage is congested. * - * * failing to actually submit a read request for a given page, + * * failing to actually submit a read request for a given folio, * possibly due to insufficient resources, or * * * getting an error during subsequent processing of a request. * - * In the last two cases, the page should be unlocked to indicate that - * the read attempt has failed. In the first case the page will be - * unlocked by the caller. + * In the last two cases, the folio should be unlocked by the filesystem + * to indicate that the read attempt has failed. In the first case the + * folio will be unlocked by the VFS. * - * Those pages not in the final ``async_size`` of the request should be + * Those folios not in the final ``async_size`` of the request should be * considered to be important and ->readahead() should not fail them due * to congestion or temporary resource unavailability, but should wait * for necessary resources (e.g. memory or indexing information) to - * become available. Pages in the final ``async_size`` may be + * become available. Folios in the final ``async_size`` may be * considered less urgent and failure to read them is more acceptable. - * In this case it is best to use delete_from_page_cache() to remove the - * pages from the page cache as is automatically done for pages that - * were not fetched with readahead_page(). This will allow a - * subsequent synchronous read ahead request to try them again. If they + * In this case it is best to use filemap_remove_folio() to remove the + * folios from the page cache as is automatically done for folios that + * were not fetched with readahead_folio(). This will allow a + * subsequent synchronous readahead request to try them again. If they * are left in the page cache, then they will be read individually using - * ->readpage(). - * + * ->readpage() which may be less efficient. */ #include @@ -157,7 +157,7 @@ static void read_pages(struct readahead_control *rac) aops->readahead(rac); /* * Clean up the remaining pages. The sizes in ->ra - * maybe be used to size next read-ahead, so make sure + * maybe be used to size next readahead, so make sure * they accurately reflect what happened. */ while ((page = readahead_page(rac))) { @@ -420,7 +420,7 @@ static pgoff_t count_history_pages(struct address_space *mapping, } /* - * page cache context based read-ahead + * page cache context based readahead */ static int try_context_readahead(struct address_space *mapping, struct file_ra_state *ra, @@ -671,9 +671,9 @@ void page_cache_sync_ra(struct readahead_control *ractl, bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM); /* - * Even if read-ahead is disabled, issue this request as read-ahead + * Even if readahead is disabled, issue this request as readahead * as we'll need it to satisfy the requested range. The forced - * read-ahead will do the right thing and limit the read to just the + * readahead will do the right thing and limit the read to just the * requested range, which we'll set to 1 page for this case. */ if (!ractl->ra->ra_pages || blk_cgroup_congested()) { @@ -689,7 +689,6 @@ void page_cache_sync_ra(struct readahead_control *ractl, return; } - /* do read-ahead */ ondemand_readahead(ractl, NULL, req_count); } EXPORT_SYMBOL_GPL(page_cache_sync_ra); @@ -697,7 +696,7 @@ EXPORT_SYMBOL_GPL(page_cache_sync_ra); void page_cache_async_ra(struct readahead_control *ractl, struct folio *folio, unsigned long req_count) { - /* no read-ahead */ + /* no readahead */ if (!ractl->ra->ra_pages) return; @@ -712,7 +711,6 @@ void page_cache_async_ra(struct readahead_control *ractl, if (blk_cgroup_congested()) return; - /* do read-ahead */ ondemand_readahead(ractl, folio, req_count); } EXPORT_SYMBOL_GPL(page_cache_async_ra);