Re: io_submit() blocks for writes for substantial amount of time

[Avi Kivity <avi@scylladb.com>]

On 09/19/2017 05:58 PM, Christoph Hellwig wrote:
> On Tue, Sep 19, 2017 at 08:27:05AM -0400, Brian Foster wrote:
>>> Please advise, is this a known bug? When can it happen? Is there a way
>>> to work it around to avoid blocking?
>>>
>> I'm not sure how either could be considered a bug based on the stack
>> trace information alone. Allocations may require reading metadata and
>> reads are synchronous. This all seems like pretty basic filesystem
>> behavior.
>>
>> I suppose performance may be a separate question. For the latter issue,
>> I'd be curious whether leaving more free space available in the
>> filesystem would help avoid running into busy extents. Perhaps having
>> more memory and thus a larger buffer cache for btree blocks could help
>> mitigate the former issue..? The deterministic workaround for both is to
>> preallocate the associated file. If the file would be too large, another
>> option may be to set an extent size hint to allocate the file in larger
>> chunks and amortize the cost of the allocations over multiple writes.
> Note that Linux 4.13 and later support a RWF_NOWAIT flag, that will
> return -EAGAIN from io_submit for these conditions so they can be
> handled by a thread pool.
>
> Note that until a few years ago we performed all allocations from
> a workqueue, this was changed by:
>
> commit cf11da9c5d374962913ca5ba0ce0886b58286224
> Author: Dave Chinner <dchinner@redhat.com>
> Date:   Tue Jul 15 07:08:24 2014 +1000
>
>      xfs: refine the allocation stack switch
>
> to only defer btree splits to a workqueue.  With that previous scheme
> there might have been an option to defer AIO allocations to a workqueue,
> but the main issue with that is that the worker thread which is then
> going to do the actual data transfer would have to "borrow" the
> mm_struct from the submitter.  That's the primary reason why something
> like that was never implemented in mainline Linux.

For DIO, does it really need the mm_struct? It can just pin the pages 
and pass them to the workqueue function.