linux-kernel - Re: [PATCH, RFC] block: set noio context in submit_bio_noacct

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Date: Thu, 25 Jan 2024 16:11:35 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: Christoph Hellwig <hch@....de>, linux-block@...r.kernel.org,
	tj@...nel.org, jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH, RFC] block: set noio context in submit_bio_noacct_nocheck

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().