From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752637Ab0FDHTL (ORCPT ); Fri, 4 Jun 2010 03:19:11 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:50267 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751409Ab0FDHTJ (ORCPT ); Fri, 4 Jun 2010 03:19:09 -0400 Date: Fri, 4 Jun 2010 08:19:06 +0100 From: Al Viro To: Borislav Petkov , Changli Gao , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] fs: optimize mpage_readpage() Message-ID: <20100604071906.GC31073@ZenIV.linux.org.uk> References: <1275095926-18345-1-git-send-email-xiaosuo@gmail.com> <20100529121019.GA25092@liondog.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100529121019.GA25092@liondog.tnic> User-Agent: Mutt/1.5.20 (2009-08-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 29, 2010 at 02:10:19PM +0200, Borislav Petkov wrote: > > - struct bio *bio = NULL; > > + struct bio *bio; > > sector_t last_block_in_bio = 0; > > struct buffer_head map_bh; > > unsigned long first_logical_block = 0; > > > > map_bh.b_state = 0; > > map_bh.b_size = 0; > > - bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio, > > + bio = do_mpage_readpage(NULL, page, 1, &last_block_in_bio, > > &map_bh, &first_logical_block, get_block); > > if (bio) > > mpage_bio_submit(READ, bio); > > Nope, I don't think that's a good idea. > > On the one hand, this is a trick to shut up gcc: > > fs/mpage.c: In function ???mpage_readpage???: > fs/mpage.c:419: warning: ???bio??? is used uninitialized in this function File a bug against your version of gcc, then. The very first operation involving bio is assignment to it; if gcc complains about that, it's extremely fscked up. Said that, I don't see how could that be an optimization. Recent gcc is apparently b0rken in dead stores elimination, but that seems to be triggered by passing address of variable to another function later on [1]; nothing of that kind happens here. [1] gcc 4.3 and later (at least) fails to eliminate the first assignment in int foo(void) { extern int f(void); extern int g(int *); int x; x = 0; x = f(); return g(&x); } with any optimization level (and apparently on any target).