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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9BBDAC433E7 for ; Mon, 31 Aug 2020 19:48:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 70FAD206F0 for ; Mon, 31 Aug 2020 19:48:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="bb8HY1CG" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729765AbgHaTsk (ORCPT ); Mon, 31 Aug 2020 15:48:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39374 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728273AbgHaTsj (ORCPT ); Mon, 31 Aug 2020 15:48:39 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 58A3EC061573; Mon, 31 Aug 2020 12:48:39 -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=lCbGSkGKUU8F1R9YT3KZ6FWEFaUZVzQp7ByD3Mg/eQQ=; b=bb8HY1CGR7xPK7g0ER8gDDtzKm sKjn7irzf13v6WwDkgC7bZ9M13Lr7/uV57MqN7evtV3T8kIwluG+Nh6586EmE6u4Zl1oe/1g5YwkZ G3BklKMwixfU1DZEB9kuG4WF9UfnokgdHuG1g7RN6dfTOYxdueQ0AA9nwxKZjF3iDZYb/nT1KpRPB eV+DiNM7n6yCr3TzsGzNN9anwQQ2eucwFJM9A2cPRmBGR8v+W2lC49CLKjhYEAFsB5PMjRxyCxFaJ i331AdMA94NOUHyFPg0J75g7EIPdNVzx1yoh/pS6FCNFPRsP4ctt3cCS+9czLdMsuBg+Guh1DK0I5 jS6KwdyA==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kCpnB-0000ZU-5D; Mon, 31 Aug 2020 19:48:37 +0000 Date: Mon, 31 Aug 2020 20:48:37 +0100 From: Matthew Wilcox To: Christoph Hellwig Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Darrick J . Wong" , linux-block@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 04/11] block: Add bio_for_each_thp_segment_all Message-ID: <20200831194837.GJ14765@casper.infradead.org> References: <20200824151700.16097-1-willy@infradead.org> <20200824151700.16097-5-willy@infradead.org> <20200827084431.GA15909@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200827084431.GA15909@infradead.org> Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Thu, Aug 27, 2020 at 09:44:31AM +0100, Christoph Hellwig wrote: > On Mon, Aug 24, 2020 at 04:16:53PM +0100, Matthew Wilcox (Oracle) wrote: > > Iterate once for each THP instead of once for each base page. > > FYI, I've always been wondering if bio_for_each_segment_all is the > right interface for the I/O completions, because we generally don't > need the fake bvecs for each page. Only the first page can have an > offset, and only the last page can be end earlier than the end of > the page size. > > It would seem way more efficient to just have a helper that extracts > the offset and end, and just use that in a loop that does the way > cheaper iteration over the physical addresses only. This might (or > might) not be a good time to switch to that model for iomap. Something like this? static void iomap_read_end_io(struct bio *bio) { int i, error = blk_status_to_errno(bio->bi_status); for (i = 0; i < bio->bi_vcnt; i++) { struct bio_vec *bvec = &bio->bi_io_vec[i]; size_t offset = bvec->bv_offset; size_t length = bvec->bv_len; struct page *page = bvec->bv_page; while (length > 0) { size_t count = thp_size(page) - offset; if (count > length) count = length; iomap_read_page_end_io(page, offset, count, error); page += (offset + count) / PAGE_SIZE; length -= count; offset = 0; } } bio_put(bio); } Maybe I'm missing something important here, but it's significantly simpler code -- iomap_read_end_io() goes down from 816 bytes to 560 bytes (256 bytes less!) iomap_read_page_end_io is inlined into it both before and after. There is some weirdness going on with regards to bv_offset that I don't quite understand. In the original bvec_advance: bv->bv_page = bvec->bv_page + (bvec->bv_offset >> PAGE_SHIFT); bv->bv_offset = bvec->bv_offset & ~PAGE_MASK; which I cargo-culted into bvec_thp_advance as: bv->bv_page = thp_head(bvec->bv_page + (bvec->bv_offset >> PAGE_SHIFT)); page_size = thp_size(bv->bv_page); bv->bv_offset = bvec->bv_offset - (bv->bv_page - bvec->bv_page) * PAGE_SIZE; Is it possible to have a bvec with an offset that is larger than the size of bv_page? That doesn't seem like a useful thing to do, but if that needs to be supported, then the code up top doesn't do that. We maybe gain a little bit by counting length down to 0 instead of counting it up to bv_len. I dunno; reading the code over now, it doesn't seem like that much of a difference. Maybe you meant something different?