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

[Ming Lei <ming.lei@redhat.com>]

On Mon, Apr 18, 2022 at 07:12:34AM +0200, Christoph Hellwig wrote:
> On Sat, Apr 16, 2022 at 05:03:35PM +0800, Ming Lei wrote:
> > > Yes.  But not doing this automatically also means you keep easily
> > > forgetting callsites.  For example iomap still does not flush the plug
> > > in your patch.
> > 
> > It is reasonable for flush user(usually submission) to be responsible
> > for finishing/flushing plug.
> 
> Well, I very much disagree here.  blk_flush_plug is not a publіc,
> exported API, and that is for a reason.  A bio submission interface
> that requires flushing the plug to be useful is rather broken.

But there isn't any such users from module now. Maybe never, since sync
polled dio becomes legacy after io_uring is invented.

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.

> 
> > 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().


Thanks,
Ming