On 08/09/2017 08:07 PM, Goldwyn Rodrigues wrote: >>>>>>> No, from a multi-device point of view, this is inconsistent. I >>>>>>> have tried the request bio returns -EAGAIN before the split, but >>>>>>> I shall check again. Where do you see this happening? >>>>>> >>>>>> No, this isn't multi-device specific, any driver can do it. >>>>>> Please see blk_queue_split. >>>>>> >>>>> >>>>> In that case, the bio end_io function is chained and the bio of >>>>> the split will replicate the error to the parent (if not already >>>>> set). >>>> >>>> this doesn't answer my question. So if a bio returns -EAGAIN, part >>>> of the bio probably already dispatched to disk (if the bio is >>>> splitted to 2 bios, one returns -EAGAIN, the other one doesn't >>>> block and dispatch to disk), what will application be going to do? >>>> I think this is different to other IO errors. FOr other IO errors, >>>> application will handle the error, while we ask app to retry the >>>> whole bio here and app doesn't know part of bio is already written >>>> to disk. >>> >>> It is the same as for other I/O errors as well, such as EIO. You do >>> not know which bio of all submitted bio's returned the error EIO. >>> The application would and should consider the whole I/O as failed. >>> >>> The user application does not know of bios, or how it is going to be >>> split in the underlying layers. It knows at the system call level. >>> In this case, the EAGAIN will be returned to the user for the whole >>> I/O not as a part of the I/O. It is up to application to try the I/O >>> again with or without RWF_NOWAIT set. In direct I/O, it is bubbled >>> out using dio->io_error. You can read about it at the patch header >>> for the initial patchset at [1]. >>> >>> Use case: It is for applications having two threads, a compute >>> thread and an I/O thread. It would try to push AIO as much as >>> possible in the compute thread using RWF_NOWAIT, and if it fails, >>> would pass it on to I/O thread which would perform without >>> RWF_NOWAIT. End result if done right is you save on context switches >>> and all the synchronization/messaging machinery to perform I/O. >>> >>> [1] http://marc.info/?l=linux-block&m=149789003305876&w=2 >> >> Yes, I knew the concept, but I didn't see previous patches mentioned >> the -EAGAIN actually should be taken as a real IO error. This means a >> lot to applications and make the API hard to use. I'm wondering if we >> should disable bio split for NOWAIT bio, which will make the -EAGAIN >> only mean 'try again'. > > Don't take it as EAGAIN, but read it as EWOULDBLOCK. Why do you say > the API is hard to use? Do you have a case to back it up? Because it is hard to use, and potentially suboptimal. Let's say you're doing a 1MB write, we hit EWOULDBLOCK for the last split. Do we return a short write, or do we return EWOULDBLOCK? If the latter, then that really sucks from an API point of view. > No, not splitting the bio does not make sense here. I do not see any > advantage in it, unless you can present a case otherwise. It ties back into the "hard to use" that I do agree with IFF we don't return the short write. It's hard for an application to use that efficiently, if we write 1MB-128K but get EWOULDBLOCK, the re-write the full 1MB from a different context. -- Jens Axboe
