Re: [PATCH] bcachefs: dio write: Take ref on mm_struct when using asynchronously

[Jens Axboe <axboe@kernel.dk>]

On 11/27/24 2:36 PM, Kent Overstreet wrote:
> When invoked from aio, mm_struct is guaranteed to outlive the request
> since its lifetime is tied to the io_context - but that's not the case
> for io_uring, it's possible that a process could be killed and mm_struct
> goes away while a request is in flight.
> 
> So if we're submitting the rest of the io asynchronously, we may need a
> ref on mm_struct.
> 
> Per Jens, this is not actually a bug because we're not yet flipping on
> FMODE_NOWAIT, meaning io_uring will do the submission from an io_worker
> kthread - but this patch is necessary for safely flipping on
> FMODE_NOWAIT for more efficient submissions in the future.

This doesn't look right. The mmgrab looks fine, but that should be
paired with mmdrop. And you still need an mmget_not_zero() before doing
kthread_use_mm(), and then the mmput/mmdrop when after unuse done.

-- 
Jens Axboe