On Tue, 2016-08-02 at 13:34 +0200, Michal Hocko wrote: > On Tue 02-08-16 12:03:23, Oliver Neukum wrote: > > On Tue, 2016-08-02 at 10:18 +0200, Michal Hocko wrote: > > > On Tue 02-08-16 10:06:12, Oliver Neukum wrote: > > > > On Mon, 2016-08-01 at 10:20 -0400, Tejun Heo wrote: > > > > > Hello, > > > > > > > > > > If any real IO depends on those devices then this is not sufficient and > > > > > > they need some form of guarantee for progress (aka mempool). > > > > > > > > > > Oliver, Alan, what do you think? If USB itself can't operate without > > > > > allocating memory during transactions, whatever USB storage drivers > > > > > > > > It cannot. The IO must be described to the hardware with a data > > > > structure in memory. > > > > > > > > > are doing isn't all that meaningful. Can we proceed with the > > > > > workqueue patches? Also, it could be that the only thing GFP_NOIO and > > > > > GFP_ATOMIC are doing is increasing the chance of IO failures under > > > > > memory pressure. Maybe it'd be a good idea to reconsider the > > > > > approach? > > > > > > > > We had actual deadlocks with GFP_KERNEL. It seems to me that the SCSI > > > > layer can deal with IO that cannot be completed due to a lack of memory > > > > at least somewhat, but a deadlock within a driver would obviously be > > > > deadly. So I don't think that mempools would remove the need for > > > > GFP_NOIO as there are places in usbcore we cannot enter the page > > > > laundering path from. They are an additional need. > > > > > > OK, I guess there is some misunderstanding here. I believe that Tejun > > > wasn't arguing to drop GFP_NOIO. It might be really needed for the dead > > > lock avoidance. No question about that. The whole point is that > > > WQ_RECLAIM might be completely pointless because a rescuer wouldn't help > > > much if the work item would do GFP_NOIO and get stuck in the page > > > allocator. > > > > But that can be a problem only if the items on the work queue are > > actually run and without WQ_MEM_RECLAIM that guarantee cannot be made. > > We can deal with failures of memory allocation. But the requests > > must actually terminate. > > I think you have missed my point. So let me ask differently. What is the > difference between your work item not running at all or looping > endlessly with GFP_NOIO inside the page allocator? If that particular > work item is necessary for the further progress then the system is > screwed one way or another. The key difference is that I could give the right parameters to the kmalloc() call. If it doesn't run, I am surely screwed. Thus I conclude that WQ_RECLAIM needs to be set. Regards Oliver -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
