On 8/17/20 2:25 AM, Stefan Metzmacher wrote: > Hi Jens, > >> Since we've had a few cases of applications not dealing with this >> appopriately, I believe the safest course of action is to ensure that >> we don't return short reads when we really don't have to. >> >> The first patch is just a prep patch that retains iov_iter state over >> retries, while the second one actually enables just doing retries if >> we get a short read back. >> >> This passes all my testing, both liburing regression tests but also >> tests that explicitly trigger internal short reads and hence retry >> based on current state. No short reads are passed back to the >> application. > > Thanks! I was going to ask about exactly that :-) > > It wasn't clear why returning short reads were justified by resulting > in better performance... As it means the application needs to do > a lot more work and syscalls. It mostly boils down to figuring out a good way to do it. With the task_work based retry, the async buffered reads, we're almost there and the prep patch adds the last remaining bits to retain the iov_iter state across issues. > Will this be backported? I can, but not really in an efficient manner. It depends on the async buffered work to make progress, and the task_work handling retry. The latter means it's 5.7+, while the former is only in 5.9+... We can make it work for earlier kernels by just using the thread offload for that, and that may be worth doing. That would enable it in 5.7-stable and 5.8-stable. For that, you just need these two patches. Patch 1 would work as-is, while patch 2 would need a small bit of massaging since io_read() doesn't have the retry parts. I'll give it a whirl just out of curiosity, then we can debate it after that. -- Jens Axboe
