Re: [PATCH RFC v4 2/3] iomap: use BIO_COMPLETE_IN_TASK for dropbehind writeback

[Gao Xiang]

On 2026/3/27 14:27, Christoph Hellwig wrote:
> On Fri, Mar 27, 2026 at 02:24:02PM +0800, Gao Xiang wrote:
> >   - use EROFS directly, in that case, we still need process
> >     contexts to decompress, but due to Android latency
> >     requirements, they really need per-cpu RT threads instead,
> >     otherwise it will cause serious regression too; but I'm not
> >     sure that case can be replaced by this work since workqueues
> >     don't support RT threads and I guess generic block layer
> >     won't be bothered with that too.
>
> All of the I/O completions should be latency sensitive.  So I think it
> would be great if you could help out here with the requirements and
> implementation.

Yes, especially for sync read completion. Our requirement can
be outlined as:

  - a mark to make the whole bio completion in task, so that
    we ensure that the bio completion is in the task context
    so that we don't need to worry about that;

  - another per-CPU RT thread flag (or similiar) relates to
    a bio or some other things, so that bio completion can be
    handled by per-cpu RT threads instead of workqueues
    instead.

If they meet, I think that would be very helpful to clean
up our internal codebase at least.

Thanks,
Gao Xiang