Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1422700AbWKPJVU (ORCPT ); Thu, 16 Nov 2006 04:21:20 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161975AbWKPJVT (ORCPT ); Thu, 16 Nov 2006 04:21:19 -0500 Received: from smtp.osdl.org ([65.172.181.4]:40602 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1031148AbWKPJVR (ORCPT ); Thu, 16 Nov 2006 04:21:17 -0500 Date: Thu, 16 Nov 2006 01:13:51 -0800 From: Andrew Morton To: cmm@us.ibm.com Cc: Hugh Dickins , Mel Gorman , "Martin J. Bligh" , linux-kernel@vger.kernel.org, "linux-ext4@vger.kernel.org" Subject: Re: Boot failure with ext2 and initrds Message-Id: <20061116011351.1401a00f.akpm@osdl.org> In-Reply-To: <1163666960.4310.40.camel@localhost.localdomain> References: <20061114014125.dd315fff.akpm@osdl.org> <20061114184919.GA16020@skynet.ie> <20061114113120.d4c22b02.akpm@osdl.org> <20061115214534.72e6f2e8.akpm@osdl.org> <455C0B6F.7000201@us.ibm.com> <20061115232228.afaf42f2.akpm@osdl.org> <1163666960.4310.40.camel@localhost.localdomain> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3678 Lines: 84 On Thu, 16 Nov 2006 00:49:20 -0800 Mingming Cao wrote: > On Wed, 2006-11-15 at 23:22 -0800, Andrew Morton wrote: > > On Wed, 15 Nov 2006 22:55:43 -0800 > > Mingming Cao wrote: > > > > > Hmm, maxblocks, in bitmap_search_next_usable_block(), is the end block > > > number of the range to search, not the lengh of the range. maxblocks > > > get passed to ext2_find_next_zero_bit(), where it expecting to take the > > > _size_ of the range to search instead... > > > > > > Something like this: (this is not a patch) > > > @@ -524,7 +524,7 @@ bitmap_search_next_usable_block(ext2_grp > > > ext2_grpblk_t next; > > > > > > - next = ext2_find_next_zero_bit(bh->b_data, maxblocks, start); > > > + next = ext2_find_next_zero_bit(bh->b_data, maxblocks-start + 1, start); > > > if (next >= maxblocks) > > > return -1; > > > return next; > > > } > > > > yes, the `size' arg to find_next_zero_bit() represents the number of bits > > to scan at `offset'. > > > > So I think your change is correctish. But we don't want the "+ 1", do we? > > > I think we still need the "+1", maxblocks here is the ending block of > the reservation window, so the number of bits to scan =end-start+1. > > > If we're right then this bug could cause the code to scan off the end of the > > bitmap. But it won't explain Hugh's bug, because of the if (next >= maxblocks). > > > > Yeah.. at first I thought it might be related, then, thinked it over, > the bug only makes the bits to scan larger, so if find_next_zero_bit() > returns something off the end of bitmap, that is fine, it just > indicating that there is no free bit left in the rest of bitmap, which > is expected behavior. So bitmap_search_next_usable_block() fail is the > expected. It will move on to next block group and try to create a new > reservation window there. I wonder why it's never oopsed. Perhaps there's always a zero in there for some reason. > That does not explain the repeated reservation window add and remove > behavior Huge has reported. I spent quite some time comparing with ext3. I'm a bit stumped and I'm suspecting that the simplistic porting the code is now OK, but something's just wrong. I assume that the while (1) loop in ext3_try_to_allocate_with_rsv() has gone infinite. I don't see why, but more staring is needed. What lock protects the fields in struct ext[234]_reserve_window from being concurrently modified by two CPUs? None, it seems. Ditto ext[234]_reserve_window_node. i_mutex will cover it for write(), but not for pageout over a file hole. If we end up with a zero- or negative-sized window then odd things might happen. > > btw, how come try_to_extend_reservation() uses spin_trylock? > > Since locks are all allocated from reservation window, when ext3 > multiple blocks allocation was added, we added try_to_extend_reservation > () to ext3, which trying to extend the reservation window size to at > least match the number of blocks to allocate. So we have better chance > to allocating multiple blocks from the window at a time. > > Since all the multiple block allocation is based on best effort basis, > the same applied to try_to_extend_reservation(). It seems no need to > wait for the reservation tree lock if it's not avaible at that moment. > I suspect that was not a good idea - it has added rather different behaviour in the once-in-a-million case. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/