Re: [PATCH V2] block: avoid io timeout in case of sync polled dio

[Christoph Hellwig <hch@lst.de>]

On Mon, Apr 18, 2022 at 04:19:09PM +0800, Ming Lei wrote:
> But there isn't any such users from module now. Maybe never, since sync
> polled dio becomes legacy after io_uring is invented.

I thought about that a bit, but if we decided synchronous polled I/O
is not in major user anymore (which I think) and we think it is enough
of a burden to support (which I'm not sure of, but this patch points to)
then it might be time to remove it.

> Do we have such potential use case in which explicit flush plug is
> needed except for polled io in __blkdev_direct_IO_simple() and swap_readpage()?
> 
> If there is, I am happy to add one flag for bypassing plug in blk core
> code.

I think the point is that we need to offer sensible APIs for I/O
methods we want to support.

> > 
> > > iomap is one good example to show this point, since it does flush the plug
> > > before call bio_poll(), see __iomap_dio_rw().
> > 
> > iomap does not do a manual plug flush anywhere.
> > 
> > iomap does finish the plug before polling, which makes sense.
> > 
> > Now of course __blkdev_direct_IO_simple doesn't even use a plug
> > to start with, so I'm wondering what plug this patch even tries
> > to flush?
>  
> At least blkdev_write_iter(), and __swap_writepage() might call
> into ->direct_IO with one plug too.
> 
> Not mention loop driver can call into ->direct_IO directly, and we
> should have applied plug for batching submission in loop_process_work().

The loop driver still calls through the read/write iter method, and
the swap code ->direct_IO path is not used for block devices (and
completley broken right now, see the patch series from Neil).

But the loop driver is a good point, even for the iomap case as the
blk_finish_plug would only flush the plug that is on-stack in
__iomap_dio_rw, it would not help you with any plug holder by caller
higher in the stack.