Re: Fixed buffer have out-dated content

Pavel Begunkov
2021-01-09T20:37:08Z

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