Messages in this thread Patch in this message | | | Subject | Re: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids | From | Anand Jain <> | Date | Sun, 25 Jul 2021 21:49:17 +0800 |
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On 22/07/2021 01:59, David Sterba wrote: > On Thu, Jul 15, 2021 at 06:34:03PM +0800, Desmond Cheong Zhi Xi wrote: >> Syzbot reports a warning in close_fs_devices that happens because >> fs_devices->rw_devices is not 0 after calling btrfs_close_one_device >> on each device. >> >> This happens when a writeable device is removed in >> __btrfs_free_extra_devids, but the rw device count is not decremented >> accordingly. So when close_fs_devices is called, the removed device is >> still counted and we get an off by 1 error. >> >> Here is one call trace that was observed: >> btrfs_mount_root(): >> btrfs_scan_one_device(): >> device_list_add(); <---------------- device added >> btrfs_open_devices(): >> open_fs_devices(): >> btrfs_open_one_device(); <-------- rw device count ++ >> btrfs_fill_super(): >> open_ctree(): >> btrfs_free_extra_devids(): >> __btrfs_free_extra_devids(); <--- device removed >> fail_tree_roots: >> btrfs_close_devices(): >> close_fs_devices(); <------- rw device count off by 1 >> >> Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device") > > What this patch did in the last hunk was the rw_devices decrement, but > conditional: > > @@ -1080,9 +1071,6 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, > if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) { > list_del_init(&device->dev_alloc_list); > clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
> - if (!test_bit(BTRFS_DEV_STATE_REPLACE_TGT, > - &device->dev_state))
This condition was wrong. The 1st roll of this patch which is here [1], has the details of why. As shown below -
[1] https://patchwork.kernel.org/project/linux-btrfs/patch/b3a0a629df98bd044a1fd5c4964f381ff6e7aa05.1600777827.git.anand.jain@oracle.com/#23640775
---- rw_devices is incremented in btrfs_open_one_device() for all write-able devices except for devid == BTRFS_DEV_REPLACE_DEVID. But while we clean up the extra devices in __btrfs_free_extra_devids() we used the BTRFS_DEV_STATE_REPLACE_TGT flag isn't set because there isn't the replace-item. So rw_devices went below zero. ----
> - fs_devices->rw_devices--; > } > list_del_init(&device->dev_list); > fs_devices->num_devices--; > --- > > >> @@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, >> if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) { >> list_del_init(&device->dev_alloc_list); >> clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state); >> + fs_devices->rw_devices--; >> } >> list_del_init(&device->dev_list); >> fs_devices->num_devices--; > > So should it be reinstated in the original form?
No. The reason is the same as above. Only the rw_devices decrement line has to be restored.
> The rest of > cf89af146b7e handles unexpected device replace item during mount.
> Adding the decrement is correct, but right now I'm not sure about the > corner case when teh devcie has the BTRFS_DEV_STATE_REPLACE_TGT bit set.
BTRFS_DEV_STATE_REPLACE_TGT is set (on BTRFS_DEV_REPLACE_DEVID) for two reasons when we call replace through ioctl or during mount upon finding a replace-device item.
> The state machine of the device bits and counters is not trivial so > fixing it one way or the other could lead to further syzbot reports if > we don't understand the issue.
I agree. Also, a good idea to convert this sysbot test into an xfstests case. Thanks, Anand
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