Re: [RFC PATCH] blk: reset 'bi_next' when bio is done inside request

[NeilBrown <neilb@suse.com>]

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On Tue, Apr 04 2017, Michael Wang wrote:

> Hi, Neil
>
> On 04/03/2017 11:25 PM, NeilBrown wrote:
>> On Mon, Apr 03 2017, Michael Wang wrote:
>> 
>>> blk_attempt_plug_merge() try to merge bio into request and chain them
>>> by 'bi_next', while after the bio is done inside request, we forgot to
>>> reset the 'bi_next'.
>>>
>>> This lead into BUG while removing all the underlying devices from md-raid1,
>>> the bio once go through:
>>>
>>>   md_do_sync()
>>>     sync_request()
>>>       generic_make_request()
>> 
>> This is a read request from the "first" device.
>> 
>>>         blk_queue_bio()
>>>           blk_attempt_plug_merge()
>>>             CHAINED HERE
>>>
>>> will keep chained and reused by:
>>>
>>>   raid1d()
>>>     sync_request_write()
>>>       generic_make_request()
>> 
>> This is a write request to some other device, isn't it?
>> 
>> If sync_request_write() is using a bio that has already been used, it
>> should call bio_reset() and fill in the details again.
>> However I don't see how that would happen.
>> Can you give specific details on the situation that triggers the bug?
>
> We have storage side mapping lv through scst to server, on server side
> we assemble them into multipath device, and then assemble these dm into
> two raid1.
>
> The test is firstly do mkfs.ext4 on raid1 then start fio on it, on storage
> side we unmap all the lv (could during mkfs or fio), then on server side
> we hit the BUG (reproducible).

So I assume the initial resync is still happening at this point?
And you unmap *all* the lv's so you expect IO to fail?
I can see that the code would behave strangely if you have a
bad-block-list configured (which is the default).
Do you have a bbl?  If you create the array without the bbl, does it
still crash?

>
> The path of bio was confirmed by add tracing, it is reused in sync_request_write()
> with 'bi_next' once chained inside blk_attempt_plug_merge().

I still don't see why it is re-used.
I assume you didn't explicitly ask for a check/repair (i.e. didn't write
to .../md/sync_action at all?).  In that case MD_RECOVERY_REQUESTED is
not set.
So sync_request() sends only one bio to generic_make_request():
   r1_bio->bios[r1_bio->read_disk];

then sync_request_write() *doesn't* send that bio again, but does send
all the others.

So where does it reuse a bio?

>
> We also tried to reset the bi_next inside sync_request_write() before
> generic_make_request() which also works.
>
> The testing was done with 4.4, but we found upstream also left bi_next
> chained after done in request, thus we post this RFC.
>
> Regarding raid1, we haven't found the place on path where the bio was
> reset... where does it supposed to be?

I'm not sure what you mean.
We only reset bios when they are being reused.
One place is in process_checks() where bio_reset() is called before
filling in all the details.


Maybe, in sync_request_write(), before

	wbio->bi_rw = WRITE;

add something like
  if (wbio->bi_next)
     printk("bi_next!= NULL i=%d read_disk=%d bi_end_io=%pf\n",
          i, r1_bio->read_disk, wbio->bi_end_io);

that might help narrow down what is happening.

NeilBrown

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