Re: [PATCH, RFC] block: set noio context in submit_bio_noacct_nocheck

[Jens Axboe <axboe@kernel.dk>]

On Thu, Jan 25, 2024 at 9:11?AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Jan 25, 2024 at 09:09:44AM -0700, Jens Axboe wrote:
> > On 1/25/24 1:10 AM, Christoph Hellwig wrote:
> > > On Wed, Jan 24, 2024 at 08:40:28AM -0700, Jens Axboe wrote:
> > >> On 1/24/24 2:39 AM, Christoph Hellwig wrote:
> > >>> Make sure all in-line block layer submission runs in noio reclaim
> > >>> context.  This is a big step towards allowing GFP_NOIO, the other
> > >>> one would be to have noio (and nofs for that matter) workqueues for
> > >>> kblockd and driver internal workqueues.
> > >>
> > >> I really don't like adding this for no good reason. Who's doing non NOIO
> > >> allocations down from this path?
> > >
> > > If there is a non-NOIO allocation right now that would be a bug,
> > > although I would not be surprised if we had a few of them.
> > >
> > > The reason to add this is a different one:  The MM folks want to
> > > get rid of GFP_NOIO and GFP_NOFS and replace them by these context.
> > >
> > > And doing this in the submission path and kblockd will cover almost
> > > all of the noio context, with the rest probably covered by other
> > > workqueues.  And this feels a lot less error prone than requiring
> > > every driver to annotate the context in their submission routines.
> >
> > I think it'd be much better to add a DEBUG protected aid that checks for
> > violating allocations. Nothing that isn't buggy should trigger this,
> > right now, and then we could catch problems if there are any. If we do
> > the save/restore there and call it good, then we're going to be stuck
> > with that forever. Regardless of whether it's actually needed or not.
>
> Nono, you don't understand.  The plan is to remove GFP_NOIO
> entirely.  Allocations should be done with GFP_KERNEL while under a
> memalloc_noio_save().

I do understand, but thanks for the vote of confidence. Place the
save/restore higher up, most likely actual IO submission isn't going to
be the only (or even major) allocation potentially needed for the IO.

-- 
Jens Axboe