Re: Fixed buffer have out-dated content

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe