Re: Fixed buffer have out-dated content

[Martin Raiber <martin@urbackup.org>]

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).

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.