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

Josef Bacik
2020-07-06T13:53:23Z

On 7/6/20 9:50 AM, Qu Wenruo wrote:
>
>
> On 2020/7/6 下午9:45, Josef Bacik wrote:
>> 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,
>
> Then we also need to handle btrfs_delalloc_reserve_metadata(),
> btrfs_block_rsv_refill(), btrfs_use_block_rsv(), btrfs_block_rsv_add().
>

This only needs to be handled for FLUSH_ALL and FLUSH_STEAL. Anybody doing
btrfs_start_transaction() should be able to fail with -EINTR, because they
should be close to the syscall path. Balance needs to be fixed to deal with it,
and I assume there might be a few other places, but by-in-large none of these
places should flip read only. Thanks,

Josef