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Mon, 06 Oct 2025 07:52:28 -0700 (PDT) Message-ID: <24f8b266-f17c-4909-b43d-8ab05721c5d8@kernel.dk> Date: Mon, 6 Oct 2025 08:52:27 -0600 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] Revert "sunvdc: Do not spin in an infinite loop when vio_ldc_send() returns EAGAIN" To: John Paul Adrian Glaubitz , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Andreas Larsson , Anthony Yznaga , Sam James , "David S . Miller" , Michael Karcher , sparclinux@vger.kernel.org References: <20251006100226.4246-2-glaubitz@physik.fu-berlin.de> <4e45e3182c4718cafad1166e9ef8dcca1c301651.camel@physik.fu-berlin.de> <5b3caa0e218dd473c8871c1b1f09a8dc1c356f1e.camel@physik.fu-berlin.de> <6ed7112cb4f338ba02d9ab67c14e7a3af4afbca0.camel@physik.fu-berlin.de> <576197870bdf21ea97559a1d84869fdcb9535156.camel@physik.fu-berlin.de> From: Jens Axboe Content-Language: en-US In-Reply-To: <576197870bdf21ea97559a1d84869fdcb9535156.camel@physik.fu-berlin.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/6/25 8:48 AM, John Paul Adrian Glaubitz wrote: > On Mon, 2025-10-06 at 08:27 -0600, Jens Axboe wrote: >>>> But that's fine, it's not uncommon for drivers to miss things like that, >>>> and then we fix them up when noticed. It was probably written by someone >>>> not super familiar with the IO stack. >>> >>> FWIW, Oracle engineers actually made some significant changes to the >>> driver that they never upstreamed, see: >>> >>> https://github.com/oracle/linux-uek/commits/uek4/qu7/drivers/block/sunvdc.c >>> >>> In particular, they added support for out-of-order execution: >>> >>> https://github.com/oracle/linux-uek/commit/68f7c9c17fb80d29cbc1e5110f6c021f8da8d610 >>> >>> and they also changed the driver to use the BIO-based interface for >>> VDC I/O requests: >>> >>> https://github.com/oracle/linux-uek/commit/4b725eb64cc10a4877f2af75ff3a776586f68eb7 >>> >>> Could you review these two changes and tell me whether these would >>> actually implement the changes you would want to see? I think the BIO >>> layer is a generic interface of the block layer in the kernel, isn't >>> it? >> >> Moving lower down the stack to use a bio directly is not a good idea, >> it's in fact going the opposite direction of what we'd like to see in >> the storage stack. And it would then mean you'd need to implement your >> own internal requeueing and retrying. > > I looked at the virtio_blk driver and that seems to confirm it. There is no > use of the bio interface either, so I guess we should not pick up this > patch. I'd be very hesitant to pick anything up that hasn't been posted and included upstream... > What do you think about the out-of-order execution? Would that make sense > to upstream it? Does it look reasonable? I have no opinion on that, there's not even a description of why that change makes any sense. Sorry but I'm not going to waste my time reviewing out-of-tree code, it's just not a very useful thing to do. If the changes get submitted upstream for review in a suitable fashion, then they will get reviewed. >> These are the kind of changes that happen when development is done and >> changes aren't submitted upstream. It's unfortunate drift... > > Well, the problem here is that Oracle stopped working on Linux for SPARC > abruptly, so many of their improvements were never sent upstream and did > not see any reviews which would have caught this. And to be frank, the changes you referenced also look pretty incomplete and would not pass upstream review. I guess they are dead in the water at this point, unless someone else picks them up and polishes them into something that can be sent upstream for review. >>>>>>> For now, I would propose to pick up my patch to revert the previous >>>>>>> change. I can then pick up your proposed change and deploy it for >>>>>>> extensive testing and see if it has any side effects. >>>>>> >>>>>> Why not just test this one and see if it works? As far as I can tell, >>>>>> it's been 6.5 years since this change went in, I can't imagine there's a >>>>>> huge sense of urgency to fix it up that can't wait for testing a more >>>>>> proper patch rather than a work-around? >>>>> >>>>> Well, the thing is that a lot of people have been running older kernels >>>>> on SPARC because of issues like these and I have started working on trying >>>>> to track down all of these issues now [2] for users to be able to run a >>>>> current kernel. So, the 6.5 years existence of this change shouldn't >>>>> be an argument I think. >>>> >>>> While I agree that the bug is unfortunate, it's also a chance to >>>> properly fix it rather than just go back to busy looping. How difficult >>>> is it to test an iteration of the patch? It'd be annoying to queue a >>>> bandaid only to have to revert that again for a real fix. If this was a >>>> regression from the last release or two then that'd be a different >>>> story, but the fact that this has persisted for 6.5 years and is only >>>> bubbling back up to mainstream now would seem to indicate that we should >>>> spend a bit of extra time to just get it right the first time. >>> >>> We could do that for sure. But I would like to hear your opinion on >>> the changes contributed by Oracle engineers first. Maybe their >>> improvements are much better so that it might make sense to try to >>> upstream them. >> >> Won't help this case, and it's actively going the wrong direction >> imho... > > OK, so your opinion is then to add the patch that you proposed on top > of what's currently there in Linus' tree, meaning adding some code > that will requeue requests once the retry limit has been reached? Right, the patch I sent is against the normal upstream tree. > Can you maybe post a proper patch then which I (and others) could test > and then hopefully add their "Tested-by"? Sure. -- Jens Axboe