On 09/01/2021 16:58, Martin Raiber wrote: > On 09.01.2021 17:23 Jens Axboe wrote: >> On 1/8/21 4:39 PM, Martin Raiber wrote: >>> Hi, >>> >>> I have a gnarly issue with io_uring and fixed buffers (fixed >>> read/write). It seems the contents of those buffers contain old data in >>> some rare cases under memory pressure after a read/during a write. >>> >>> Specifically I use io_uring with fuse and to confirm this is not some >>> user space issue let fuse print the unique id it adds to each request. >>> Fuse adds this request data to a pipe, and when the pipe buffer is later >>> copied to the io_uring fixed buffer it has the id of a fuse request >>> returned earlier using the same buffer while returning the size of the >>> new request. Or I set the unique id in the buffer, write it to fuse (via >>> writing to a pipe, then splicing) and then fuse returns with e.g. >>> ENOENT, because the unique id is not correct because in kernel it reads >>> the id of the previous, already completed, request using this buffer. >>> >>> To make reproducing this faster running memtester (which mlocks a >>> configurable amount of memory) with a large amount of user memory every >>> 30s helps. So it has something to do with swapping? It seems to not >>> occur if no swap space is active. Problem occurs without warning when >>> the kernel is build with KASAN and slab debugging. >>> >>> If I don't use the _FIXED opcodes (which is easy to do), the problem >>> does not occur. >>> >>> Problem occurs with 5.9.16 and 5.10.5. >> Can you mention more about what kind of IO you are doing, I'm assuming >> it's O_DIRECT? I'll see if I can reproduce this. > > It's writing to/reading from pipes (nonblocking, no O_DIRECT). A blind guess, does it handle short reads and writes? If not, can you check whether they happen or not? > > I can reproduce it with https://github.com/uroni/fuseuring on e.g. a 2GB VPS. Modify bench.sh so that fio loops. Add swap, then run 1400M memtester while it runs (so it swaps, I guess). I can try further reducing the reproducer, but I wanted to avoid that work in case it is something obvious. The next step would be to remove fuse from the equation -- it does try to move the pages from the pipe when splicing to it, for example. -- Pavel Begunkov
