From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: with ECARTIS (v1.0.0; list xfs); Sun, 22 Jun 2008 23:13:28 -0700 (PDT) Received: from cuda.sgi.com ([192.48.176.15]) by oss.sgi.com (8.12.11.20060308/8.12.11/SuSE Linux 0.7) with ESMTP id m5N6DQ3m031957 for ; Sun, 22 Jun 2008 23:13:26 -0700 Received: from ipmail01.adl6.internode.on.net (localhost [127.0.0.1]) by cuda.sgi.com (Spam Firewall) with ESMTP id 0DD6B180AEEF for ; Sun, 22 Jun 2008 23:14:25 -0700 (PDT) Received: from ipmail01.adl6.internode.on.net (ipmail01.adl6.internode.on.net [203.16.214.146]) by cuda.sgi.com with ESMTP id N9wuRbF5Cxs473PX for ; Sun, 22 Jun 2008 23:14:25 -0700 (PDT) Date: Mon, 23 Jun 2008 16:14:21 +1000 From: Dave Chinner Subject: Re: [PATCH] Prevent extent btree block allocation failures Message-ID: <20080623061421.GG29319@disturbed> References: <485223E4.6030404@sgi.com> <20080613155708.GG3700@disturbed> <485603FD.2080204@sgi.com> <200806161010.22476.dchinner@agami.com> <48571A57.5090901@sgi.com> <20080617073949.GP3700@disturbed> <485A0AB2.4060009@sgi.com> <20080620052120.GA3700@disturbed> <20080623052025.GF29319@disturbed> <485F3B42.9050300@sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <485F3B42.9050300@sgi.com> Sender: xfs-bounce@oss.sgi.com Errors-to: xfs-bounce@oss.sgi.com List-Id: xfs To: Lachlan McIlroy Cc: xfs-dev , xfs-oss On Mon, Jun 23, 2008 at 03:57:22PM +1000, Lachlan McIlroy wrote: > Dave Chinner wrote: >> On Fri, Jun 20, 2008 at 03:21:20PM +1000, Dave Chinner wrote: >>> On Thu, Jun 19, 2008 at 05:28:50PM +1000, Lachlan McIlroy wrote: >>>> There's something else that looks suspicious to me - this code in >>>> xfs_bmap_btalloc() is setting minleft to 0. Doesn't this go against >>>> what you were saying about setting minleft to be the space we might >>>> need for the btree operations? >>>> >>>> if (args.fsbno == NULLFSBLOCK && nullfb) { >>>> args.fsbno = 0; >>>> args.type = XFS_ALLOCTYPE_FIRST_AG; >>>> args.total = ap->minlen; >>>> args.minleft = 0; >>>> if ((error = xfs_alloc_vextent(&args))) >>>> return error; >>>> ap->low = 1; >>>> } >>> Hmmm - that looks suspicious. In xfs_bmapi(), when we are doing a >>> write and *firstblock == NULLFSBLOCK (which leads to nullfb being >>> set in the above code), we do: >>> >>> if (wr && *firstblock == NULLFSBLOCK) { >>> if (XFS_IFORK_FORMAT(ip, whichfork) == XFS_DINODE_FMT_BTREE) >>> minleft = be16_to_cpu(ifp->if_broot->bb_level) + 1; >>> else >>> minleft = 1; >>> } else >>> minleft = 0; >>> >>> If we are in btree format we set the minleft to the number of blocks needed >>> for a split. If we are in extent or local format, change to extent of btree >>> format requires one extra block. >>> >>> The above code you point out definitely breaks this - we haven't done a >>> previous allocation so we can start from the first AG, but we sure as >>> hell still need minleft set to the number of blocks needed for a >>> format change or btree split. >> >> Just to point out yet another problem in this code (one that's just >> been tripped over @ agami) is unwritten extent conversion. >> >> Basically, we don't do an allocation here, so when we end up in >> xfs_bmap_add_extent_unwritten_real() with a null firstblock. Hence >> the cases where conversion can cause a split - case >> MASK(LEFT_FILLING), MASK(RIGHT_FILLING) and 0 (convert the middle of >> an extent) - we can select an AG that doesn't have enough space for >> the entire split as we've ignored the number of blocks we might >> need to allocate in the split (the minleft parameter) entirely. >> >> I suspect that xfs_bmbt_split() needs to handle the null first block >> case slightly differently - the minleft parameter passed to the >> allocation should not be zero - it should be the number of levels >> above the current level left in the tree. i.e: >> >> minleft = be16_to_cpu(ifp->if_broot->bb_level) + 1; >> >> If we've already got a firstblock set, then this should have already >> been taken into account (i.e. we still need to fix the low space >> case where it got ignored as we were discussing). > > Funny. I tested the exact same change last week to try to fix the same > problem. Seemed to work okay. Cool. Got a patch for review? > In the case where we convert the middle of an existing unwritten extent > we need to insert two new extents. I might be paranoid here but I'll > assume the worst case scenario and that we'll need space for two complete > tree splits. Yes, I think so. Certainly, if you look at the block reservation in xfs_iomap_write_unwritten(): 892 resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1; #define XFS_DIOSTRAT_SPACE_RES(mp, v) \ (XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK) + (v)) #define XFS_EXTENTADD_SPACE_RES(mp,w) (XFS_BM_MAXLEVELS(mp,w) - 1) It reserves enough blocks for 2 bmbt splits so I think this is definitely a possibility we need to handle. > The first allocation for the first insert will set minleft > correctly but what about the allocations for splits during the second > insert? We could run out of space in the chosen AG because minleft wasn't > enough. Yeah, so we probably need pass a flag in the cursor to indicate it's a double split case when doing the first allocation in xfs_bmbt_split.... Cheers, Dave. -- Dave Chinner david@fromorbit.com