On 21.06.2018 [15:51:08 +0200], Kevin Wolf wrote: > Am 21.06.2018 um 05:26 hat Nishanth Aravamudan geschrieben: > > On 20.06.2018 [12:34:52 -0700], Nishanth Aravamudan wrote: > > > On 20.06.2018 [11:57:42 +0200], Kevin Wolf wrote: > > > > Am 20.06.2018 um 00:54 hat Nishanth Aravamudan geschrieben: > > > > > On 19.06.2018 [15:35:57 -0700], Nishanth Aravamudan wrote: > > > > > > On 19.06.2018 [13:14:51 -0700], Nishanth Aravamudan wrote: > > > > > > > On 19.06.2018 [14:35:33 -0500], Eric Blake wrote: > > > > > > > > On 06/15/2018 12:47 PM, Nishanth Aravamudan via Qemu-devel > > > > > > > > wrote: > > > > > > > > > > > > <snip> > > > > > > > > > > > > > > > } else if (s->use_linux_aio) { > > > > > > > > > + int rc; > > > > > > > > > + rc = > > > > > > > > > aio_setup_linux_aio(bdrv_get_aio_context(bs)); > > > > > > > > > + if (rc != 0) { > > > > > > > > > + error_report("Unable to use native AIO, > > > > > > > > > falling back to " > > > > > > > > > + "thread pool."); > > > > > > > > > > > > > > > > In general, error_report() should not output a trailing '.'. > > > > > > > > > > > > > > Will fix. > > > > > > > > > > > > > > > > + s->use_linux_aio = 0; > > > > > > > > > + return rc; > > > > > > > > > > > > > > > > Wait - the message claims we are falling back, but the non-zero > > > > > > > > return code > > > > > > > > sounds like we are returning an error instead of falling back. > > > > > > > > (My > > > > > > > > preference - if the user requested something and we can't do > > > > > > > > it, it's better > > > > > > > > to error than to fall back to something that does not match the > > > > > > > > user's > > > > > > > > request). > > > > > > > > > > > > > > I think that makes sense, I hadn't tested this specific case (in > > > > > > > my > > > > > > > reading of the code, it wasn't clear to me if raw_co_prw() could > > > > > > > be > > > > > > > called before raw_aio_plug() had been called, but I think > > > > > > > returning the > > > > > > > error code up should be handled correctly. What about the cases > > > > > > > where > > > > > > > there is no error handling (the other two changes in the patch)? > > > > > > > > > > > > While looking at doing these changes, I realized that I'm not quite > > > > > > sure > > > > > > what the right approach is here. My original rationale for returning > > > > > > non-zero was that AIO was requested but could not be completed. I > > > > > > haven't fully tracked back the calling paths, but I assumed it > > > > > > would get > > > > > > retried at the top level, and since we indicated to not use AIO on > > > > > > subsequent calls, it will succeed and use threads then (note, that > > > > > > I do > > > > > > now realize this means a mismatch between the qemu command-line and > > > > > > the > > > > > > in-use AIO model). > > > > > > > > > > > > In practice, with my v2 patch, where I do return a non-zero > > > > > > error-code > > > > > > from this function, qemu does not exit (nor is any logging other > > > > > > than > > > > > > that I added emitted on the monitor). If I do not fallback, I > > > > > > imagine we > > > > > > would just continuously see this error message and IO might not > > > > > > actually > > > > > > every occur? Reworking all of the callpath to fail on non-zero > > > > > > returns > > > > > > from raw_co_prw() seems like a fair bit of work, but if that is > > > > > > what is > > > > > > being requested, I can try that (it will just take a while). > > > > > > Alternatively, I can produce a v3 quickly that does not bubble the > > > > > > actual errno all the way up (since it does seem like it is ignored > > > > > > anyways?). > > > > > > > > > > Sorry for the noise, but I had one more thought. Would it be > > > > > appropriate > > > > > to push the _setup() call up to when we parse the arguments about > > > > > aio=native? E.g., we already check there if cache=directsync is > > > > > specified and error out if not. > > > > > > > > We already do this: > > > > > > Right, I stated above it already is done, I simply meant adding a second > > > check here that we can obtain and setup the AIO context successfully. > > Yes, sorry, I misread. > > > > > /* Currently Linux does AIO only for files opened with O_DIRECT */ > > > > if (s->use_linux_aio && !(s->open_flags & O_DIRECT)) { > > > > error_setg(errp, "aio=native was specified, but it requires " > > > > "cache.direct=on, which was not specified."); > > > > ret = -EINVAL; > > > > goto fail; > > > > } > > > > > > > > laio_init() is about other types of errors. But anyway, yes, calling > > > > laio_init() already in .bdrv_open() is possible. Returning errors from > > > > .bdrv_open() is nice and easy and we should do it. > > > > > > Ack. > > > > > > > However, we may also need to call laio_init() again when switching to a > > > > different I/O thread after the image is already opened. This is what I > > > > meant when I commented on v1 that you should do this in the > > > > .bdrv_attach_aio_context callback. The problem here is that we can't > > > > return an error there and the guest is already using the image. In this > > > > case, logging an error and falling back to the thread pool seems to be > > > > the best option we have. > > > > > > Is this is a request for new functionality? Just trying to understand, > > > because aiui, block/file-posix.c does not implement the > > > bdrv_attach_aio_context callback currently. Instead, aio_get_linux_aio() > > > is called from three places, raw_co_prw, raw_aio_plug and > > > raw_aio_unplug, which calls into laio_init() and > > > laio_attach_aio_context(). I can add the callback you suggest with > > > appropriate error handling (I suppose it would point to > > > laio_attach_aio_context, possibly with some modifications) and remove > > > the call from aio_get_linux_aio()? Just trying to understand the request > > > a bit better, as I don't see where exactly iothreads get switched and > > > how that is implemented currently (and thus where laio_init() would get > > > called again in the current code). > > It's not new functionality, but moving things to a different place. > > Our goal is that aio_get_linux_aio() never returns NULL in the I/O path > so that we don't have to duplicate the error handling everywhere. For > each AioContext, aio_get_linux_aio() can return NULL only the first > time. Once it successfully returned a LinuxAioState, it will never > return NULL again for the same AioContext. > > You cover the QEMU main loop AioContext when you call > aio_get_linux_aio() in raw_open_common(). But whenever the AioContext is > changed, we can get NULL again on the next aio_get_linux_aio() call > because we don't know whether that AioContext already successfully > returned a LinuxAioState before. > > If you call once it in .bdrv_attach_aio_context() and disable Linux AIO > if it fails, you have centralised all the error paths that you currently > need in all I/O paths.
Thanks for the clarification! > > While I waited for a reply to this, I started coding on what I think was > > being asked for and have come to the conclusion that there are actually > > three bugs here :) > > > > Test cases (with one disk attached to the VM): > > > > 1) Set /proc/sys/fs/max-aio-nr to 0. Specify aio=native and qemu dies > > with a SIGSEGV. > > - This case is understood and pushing the laio_init()-return code > > check to the bdrv_open() path fixes this (and allows for the > > failure to be communicated to the user). > > Right. > > > 2) Set /proc/sys/fs/max-aio-nr to 128. Specify aio=native and some > > number of IOThreads. Over qmp issue a x-blockdev-set-iothread command to > > move the block device node to one of the IOThreads. qemu eventually dies > > with a SIGSEGV. > > - I am fairly sure this is the case you described above, and is > > fixed by re-implementing the bdrv_{attach,detach}_aio_context > > callbacks. I have a patch that does this and successfully tested > > the SEGV is avoided. > > > > 3) Set /proc/sys/fs/max-aio-nr to 512 (I think 256 would be sufficient, > > though). Specify aio=native and some number of IOThreads. Over qmp issue > > a x-blockdev-set-iothread command to move the block device node to one > > of the IOThreads. Shutdown the guest normally. qemu dies with a SIGABRT. > > - This appears to be because there is a mismatch in > > aio_context_{acquire,release} calls (this is my hypothesis right > > now). The abort comes from bdrv_flush -> aio_context_release and > > an EPERM from qemu_mutex_unlock_impl() which I believe is just > > reflecting an EPERM from pthread_mutex_unlock? My theory is that > > the main qemu thread acquired the aio mutex but then the IOThread > > released it? I will try and trace the mutexes tomorrow, but I > > still don't have a fix for this case. > > x-blockdev-set-iothread is not the proper way to achieve what you want. > You can only correctly use it with images that are not attached to any > guest device. If you move a block node to a different I/O thread (and > therefore AioContext) under the feet of the guest device, crashes aren't > completely unexpected. > > The proper way to test this would be -device virtio-blk,iothread=... > In this case, the raw BlockDriverState is first created in the main > AioContext, but when the virtio-blk device is initialised, it moves it > to the specified I/O thread. Thanks again! Turns out 2) is definitely present in master. 3) does not occur. So I will work on making my patch into 2 patches and repost a v3 later today. Thanks, Nish