linux-kernel - Re: [PATCH] ext4: fix indirect punch hole corruption

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Date:	Thu, 5 Feb 2015 13:41:10 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Chris J Arges <chris.j.arges@...onical.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: fix indirect punch hole corruption

On Thu, Feb 05, 2015 at 03:30:47PM -0600, Chris J Arges wrote:
> On 02/05/2015 02:50 PM, Omar Sandoval wrote:
> > Commit 4f579ae7de56 (ext4: fix punch hole on files with indirect
> > mapping) rewrote FALLOC_FL_PUNCH_HOLE for ext4 files with indirect
> > mapping. However, the case where the punch happens within one level of
> > indirection is incorrect. It assumes that the partial branches returned
> > from ext4_find_shared will have the same depth, but this is not
> > necessarily the case even when the offsets have the same depth. For
> > example, if the last block occurs at the beginning of an indirect group
> > (i.e., it has an offset of 0 at the end of the offsets array), then
> > ext4_find_shared will return a shallower chain. So, let's handle the
> > mismatch and clean up that case. Tested with generic/270, which no
> > longer leads to an inconsistent filesystem like before.
> > 
> > Signed-off-by: Omar Sandoval <osandov@...ndov.com>
> 
> 
> Omar,
> 
> Tried running this with my original reproducer (qcow2 snapshotting and
> rebooting) and got the following:
> ------------[ cut here ]------------
> kernel BUG at fs/ext4/indirect.c:1488!
> invalid opcode: 0000 [#1] SMP
> <snip>
> CPU: 4 PID: 9771 Comm: qemu-img Tainted: G        W   E
> 3.19.0-rc7-b164aa5 #22
> Hardware name: XXX
> task: ffff880243a34aa0 ti: ffff880240f3c000 task.ti: ffff880240f3c000
> RIP: 0010:[<ffffffff812a38e7>]  [<ffffffff812a38e7>]
> ext4_ind_remove_space+0x737/0x740
> RSP: 0018:ffff880240f3fc98  EFLAGS: 00010246
> RAX: ffff880240f3fd98 RBX: ffff880240f3fd98 RCX: ffff880098c684dc
> RDX: ffff880098c684d4 RSI: ffff880240f3fd08 RDI: ffff880098c684e0
> RBP: ffff880240f3fdf8 R08: ffff880098c684e0 R09: 0000000000000000
> R10: ffff880240f3faa0 R11: 0000000000000038 R12: ffff88009bb65810
> R13: 0000000000000003 R14: ffff880240f3fd08 R15: ffff880240f3fd68
> FS:  00007f7ad84ad700(0000) GS:ffff88024e500000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7ae19a78ff CR3: 0000000241c52000 CR4: 00000000003427e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Stack:
>  ffffea0002596600 ffff88009bb65960 0000000000000050 0000000000000100
>  ffff8802447a2900 0000000000000003 ffff880240f3fd08 ffff88009b96c030
>  ffff880240f3fd08 000000000000024b 000000000000000d ffff880000000034
> Call Trace:
>  [<ffffffff8129ccdc>] ? ext4_discard_preallocations+0x16c/0x480
>  [<ffffffff8126982f>] ext4_punch_hole+0x3bf/0x430
>  [<ffffffff81293c9e>] ext4_fallocate+0x16e/0x8c0
>  [<ffffffff811e4849>] ? __sb_start_write+0x49/0xf0
>  [<ffffffff811df3cf>] vfs_fallocate+0x12f/0x250
>  [<ffffffff810eda41>] ? SyS_futex+0x71/0x150
>  [<ffffffff811e0408>] SyS_fallocate+0x48/0x80
>  [<ffffffff8177cc2d>] system_call_fastpath+0x16/0x1b
> Code: 40 4c 8d 0c c5 e8 ff ff ff 49 c1 f9 03 45 69 c9 ab aa aa aa e8 6b
> e2 ff ff 48 8b 85 10 ff ff ff c7 00 00 00 00 00 e9 fd fa ff ff <0f> 0b
> 0f 0b 0f 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 41 56 41
> RIP  [<ffffffff812a38e7>] ext4_ind_remove_space+0x737/0x740
>  RSP <ffff880240f3fc98>
> ---[ end trace 05f053fdd5d908a8 ]---
> 
> So this is hitting the BUG_ON you added.
> --chris
> 
Ah, yes, thanks Chris, my logic there was wrong. For example, punching
from the beginning of the first doubly-indirected block to, say, the
100th doubly-indirected block, will hit this. Gimme a sec and I'll fix
it.

-- 
Omar
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