Re: [PATCH] Btrfs: fix handling of faults from btrfs_copy_from_user

[Adam Borowski <kilobyte@angband.pl>]

On Tue, May 17, 2016 at 05:00:49PM -0400, Chris Mason wrote:
> On Tue, May 17, 2016 at 10:47:27PM +0200, Adam Borowski wrote:
> > > Hmmm, some of your traces mentioned compression, do you have compression
> > > enabled?
> > 
> > Yeah, I mount with noatime,compress=lzo.
> > 
> > > Also, if you can gdb your vmlinux (or btrfs.ko) and find this line:
> > > 
> > > gdb> list *__btrfs_buffered_write+0x748
> > 
> > 0xffffffff8152eb78 is in __btrfs_buffered_write (fs/btrfs/file.c:1564).
> > 1559	
> > 1560	reserve_metadata:
> > 1561			ret = btrfs_delalloc_reserve_metadata(inode, reserve_bytes);
> > 1562			if (ret) {
> > 1563				if (!only_release_metadata)
> > 1564					btrfs_free_reserved_data_space(inode, pos,
> > 1565								       write_bytes);
> > 1566				else
> > 1567					btrfs_end_write_no_snapshoting(root);
> > 1568				break;
> > 
> 
> Aha, so you're getting an enospc with compression on and the accounting
> doesn't line up.  I should be able to trigger this.

This partition was fully balanced just a few days ago so I didn't even think
to look at df.

[~]# df -h
/dev/sda1       216G  161G   53G  76% /

[~]# btrfs files df /
Data, single: total=209.12GiB, used=156.46GiB
System, single: total=32.00MiB, used=48.00KiB
Metadata, single: total=6.00GiB, used=3.51GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

However, after partially balancing it a bit:

[~]# btrfs files df /
Data, single: total=172.00GiB, used=156.03GiB
System, single: total=32.00MiB, used=48.00KiB
Metadata, single: total=5.00GiB, used=3.50GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

I can't reproduce the failure anymore.

-- 
A tit a day keeps the vet away.