From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hugh Dickins Subject: Re: [patch] sg: simplify page_count manipulations Date: Fri, 20 Jan 2006 16:32:00 +0000 (GMT) Message-ID: References: <20060118155242.GB28418@wotan.suse.de> <20060118195937.3586c94f.akpm@osdl.org> <20060119144548.GF958@wotan.suse.de> <20060119140525.223a8ebf.akpm@osdl.org> <20060120101815.GD1756@wotan.suse.de> <20060120024702.6f894a13.akpm@osdl.org> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Return-path: Received: from silver.veritas.com ([143.127.12.111]:50957 "EHLO silver.veritas.com") by vger.kernel.org with ESMTP id S1751053AbWATQcB (ORCPT ); Fri, 20 Jan 2006 11:32:01 -0500 Received: from goblin.wat.veritas.com([10.10.97.2]) (3119 bytes) by megami.veritas.com via sendmail with P:esmtp/R:smart_host/T:smtp (sender: ) id for ; Fri, 20 Jan 2006 08:31:25 -0800 (PST) (Smail-3.2.0.101 1997-Dec-17 #15 built 2001-Aug-30) In-Reply-To: <20060120024702.6f894a13.akpm@osdl.org> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Andrew Morton Cc: Nick Piggin , torvalds@osdl.org, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, dougg@torque.net On Fri, 20 Jan 2006, Andrew Morton wrote: > Nick Piggin wrote: > > On Thu, Jan 19, 2006 at 02:05:25PM -0800, Andrew Morton wrote: > > > Nick Piggin wrote: > > > > > > > > On Wed, Jan 18, 2006 at 07:59:37PM -0800, Andrew Morton wrote: > > > > > Nick Piggin wrote: > > > > > > - /* N.B. correction _not_ applied to base page of each allocation */ > > > > > > - for (k = 0; k < rsv_schp->k_use_sg; ++k, ++sg) { > > > > > > - for (m = PAGE_SIZE; m < sg->length; m += PAGE_SIZE) { > > > > > > - page = sg->page; > > > > > > - if (startFinish) > > > > > > - get_page(page); > > > > > > - else { > > > > > > - if (page_count(page) > 0) > > > > > > - __put_page(page); > > > > > > - } > > > > > > - } > > > > > > - } > > > > > > -} > > > > > > > > > > What on earth is the above trying to do? The inner loop is a rather > > > > > complex way of doing atomic_add(&page->count, sg->length/PAGE_SIZE). One > > > > > suspects there's a missing "[m]" in there. > > > > > > > > > > > > > It does this on the first mmap of the device, in the hope that subsequent > > > > nopage, unmaps would not free the constituent pages in the scatterlist. > > > > > > > > > > But it's doing it wrongly, isn't it? Or am I completely nuts? > > > > No I think you're right. I'm not sure why this doesn't oops but I > > thought it was the (main) reason others wanted to get rid of this > > convoluted code earlier on. I see nobody else is planning to do anything > > about it though, so I figure I must have missed the reason why it isn't > > a problem. > > > > But either way I don't think the code actually _does_ anything, even if > > its bugginess doesn't actually lead to a bug. > > > > I suspect nobody tried to munmap pages beyond the first one. > > Yes, let's use a compound page in there and I expect Doug will be able to > test it for us sometime. That function did move page along in 2.6.15, but has got screwed up since: good reason, I think, to speed Nick's patch through to clean it all away. Hugh