Re: calling into file systems directly from ->queue_rq, was Re: [PATCH V5 0/6] loop: improve loop aio perf by IOCB_NOWAIT

[Ming Lei <ming.lei@redhat.com>]

On Sun, Nov 23, 2025 at 10:12:24PM -0800, Christoph Hellwig wrote:
> FYI, with this series I'm seeing somewhat frequent stack overflows when
> using loop on top of XFS on top of stacked block devices.

Can you share your setting?

BTW, there are one followup fix:

https://lore.kernel.org/linux-block/20251120160722.3623884-1-ming.lei@redhat.com/

I just run 'xfstests -q quick' on loop on top of XFS on top of dm-stripe,
not see stack overflow with the above fix against -next.

> 
> This seems to be because this can now issue I/O directly from ->queue_rq
> instead of breaking the stack chain, i.e. we can build much deeper call
> stacks now.
> 
> Also this now means a file systems using current->journal_info can call
> into another file system trying to use, making things blow up even worse.
> 
> In other words:  I don't think issuing file system I/O from the
> submission thread in loop can work, and we should drop this again.

I don't object to drop it one more time.

However, can we confirm if it is really a stack overflow because of
calling into FS from ->queue_rq()?

If yes, it could be dead end to improve loop in this way, then I can give up.


Thanks, 
Ming