Re: btrfs BUG() in __set_extent_bit on GFP_ATOMIC failure.

[Chris Mason <clm@fb.com>]

On 08/08/2014 12:26 AM, Dave Jones wrote:
> While playing with fault injection, I hit this quite easily.
> 
> kernel BUG at fs/btrfs/extent_io.c:990!
> invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> CPU: 1 PID: 1270 Comm: fsx Not tainted 3.16.0+ #41
> task: ffff88023fe46d60 ti: ffff8802405a8000 task.ti: ffff8802405a8000
> RIP: 0010:[<ffffffffc0319af4>]  [<ffffffffc0319af4>] __set_extent_bit+0x574/0x660 [btrfs]
> ...
>  [<ffffffff851be7fc>] ? set_track+0x9c/0x140
>  [<ffffffffc031aa94>] lock_extent_bits+0x94/0x310 [btrfs]
>  [<ffffffff85166454>] ? pagecache_get_page+0xb4/0x210
>  [<ffffffffc030d4de>] lock_and_cleanup_extent_if_need+0xee/0x1f0 [btrfs]
>  [<ffffffffc030e6e1>] __btrfs_buffered_write+0x1b1/0x680 [btrfs]
>  [<ffffffff850a258b>] ? preempt_count_sub+0xab/0x100
>  [<ffffffffc030ed2e>] btrfs_file_write_iter+0x17e/0x570 [btrfs]
>  [<ffffffff851d78ce>] new_sync_write+0x8e/0xd0
>  [<ffffffff851d8127>] vfs_write+0xb7/0x1f0
>  [<ffffffff851d8d68>] SyS_write+0x58/0xd0
>  [<ffffffff8576371f>] tracesys+0xdd/0xe2
> 
> 
>  989                 prealloc = alloc_extent_state_atomic(prealloc);
>  990                 BUG_ON(!prealloc);
> 
>  541 static struct extent_state *
>  542 alloc_extent_state_atomic(struct extent_state *prealloc)
>  543 {
>  544         if (!prealloc)
>  545                 prealloc = alloc_extent_state(GFP_ATOMIC);
>  546 
>  547         return prealloc;
>  548 }
> 
> 
> Going BUG() on a GFP_ATOMIC allocation failure seems a bit excessive.
> Surely there's something better we can do here ?
> 

Ugh, yes we can.  It should jump back to outside the lock and get the
prealloc there.

-chris