Re: [PATCH RFC 2/2] btrfs: space-info: Don't allow signal to interrupt ticket waiting

[Josef Bacik <josef@toxicpanda.com>]

On 7/6/20 3:44 AM, Qu Wenruo wrote:
> [BUG]
> When balance receive a fatal signal, it can make the fs to read-only
> mode if the timing is unlucky enough:
> 
>    BTRFS info (device xvdb): balance: start -d -m -s
>    BTRFS info (device xvdb): relocating block group 73001861120 flags metadata
>    BTRFS info (device xvdb): found 12236 extents, stage: move data extents
>    BTRFS info (device xvdb): relocating block group 71928119296 flags data
>    BTRFS info (device xvdb): found 3 extents, stage: move data extents
>    BTRFS info (device xvdb): found 3 extents, stage: update data pointers
>    BTRFS info (device xvdb): relocating block group 60922265600 flags metadata
>    BTRFS: error (device xvdb) in btrfs_drop_snapshot:5505: errno=-4 unknown
>    BTRFS info (device xvdb): forced readonly
>    BTRFS info (device xvdb): balance: ended with status: -4
> 
> [CAUSE]
> This is caused by the fact that btrfs ticketing space system can be
> interrupted, and cause all kind of -EINTR returned to various critical
> section, where we never thought of -EINTR at all.
> 
> Even for things like btrfs_start_transaction() can be affected by
> signal:
>   btrfs_start_transaction()
>   |- start_transaction(flush = FLUSH_ALL)
>      |- btrfs_block_rsv_add()
>         |- btrfs_reserve_metadata_bytes()
>            |- __reserve_metadata_bytes()
>               |- handle_reserve_ticket()
>                  |- wait_reserve_ticket()
>                     |- prepare_to_wait_event(TASK_KILLABLE)
>                     |- ticket->error = -EINTR;
> 
> And all related callers get -EINTR error.
> 
> In fact, there are really very limited call sites can really handle that
> -EINTR properly, above btrfs_drop_snapshot() is one case.
> 
> [FIX]
> Things like metadata allocation is really a critical section for btrfs,
> we don't really want it to be that killable by some impatient users.
> 
> In fact, for really long duration calls, it should have their own checks
> on signal, like balance, reflink, generic fiemap calls.
> 
> So this patch will make ticket waiting uninterruptible, relying on each
> long duration calls to handle their signals more properly.
> 

Nope, everybody that calls start_transaction() should be able to handle -EINTR, 
so we need to find those callsites and fix them, not make it so we hang the box 
because we don't like fixing error paths.  Thanks,

Josef