Re: Fixed buffer have out-dated content

[Martin Raiber <martin@urbackup.org>]

On 09.01.2021 21:32 Pavel Begunkov wrote:
> 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?

Something like this was what I suspected at first as well. It does check 
for short read/writes and I added (unnecessary -- because the fuse 
request structure is 40 bytes and it does io in page sizes) code for 
retrying short reads at some point. I also checked for the pipes to be 
empty before they are used at some point and let the kernel log 
allocation failures (idea was that it was short pipe read/writes because 
of allocation failure or that something doesn't get rewound properly in 
this case). Beyond that three things that make a user space problem 
unlikely:

  - occurs only when using fixed buffers and does not occur when running 
same code without fixed buffer opcodes
  - doesn't occur when there is no memory pressure
  - I added print(k/f) logging that pointed me in this direction as well

>> 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.
>