linux-kernel - Re: [PATCH 2/3] xfs: add kmem_alloc

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Date:   Thu, 22 Aug 2019 11:10:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Dave Chinner <david@...morbit.com>, linux-xfs@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        penguin-kernel@...ove.SAKURA.ne.jp
Subject: Re: [PATCH 2/3] xfs: add kmem_alloc_io()

On Thu, Aug 22, 2019 at 10:51:30AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 22, 2019 at 12:59:48AM -0700, Christoph Hellwig wrote:
> > On Thu, Aug 22, 2019 at 10:31:32AM +1000, Dave Chinner wrote:
> > > > Btw, I think we should eventually kill off KM_NOFS and just use
> > > > PF_MEMALLOC_NOFS in XFS, as the interface makes so much more sense.
> > > > But that's something for the future.
> > > 
> > > Yeah, and it's not quite as simple as just using PF_MEMALLOC_NOFS
> > > at high levels - we'll still need to annotate callers that use KM_NOFS
> > > to avoid lockdep false positives. i.e. any code that can be called from
> > > GFP_KERNEL and reclaim context will throw false positives from
> > > lockdep if we don't annotate tehm correctly....
> > 
> > Oh well.  For now we have the XFS kmem_wrappers to turn that into
> > GFP_NOFS so we shouldn't be too worried, but I think that is something
> > we should fix in lockdep to ensure it is generally useful.  I've added
> > the maintainers and relevant lists to kick off a discussion.
> 
> Strictly speaking the fs_reclaim annotation is no longer part of the
> lockdep core, but is simply a fake lock in page_alloc.c and thus falls
> under the mm people's purview.
> 
> That said; it should be fairly straight forward to teach
> __need_fs_reclaim() about PF_MEMALLOC_NOFS, much like how it already
> knows about PF_MEMALLOC.

Ah, current_gfp_context() already seems to transfer PF_MEMALLOC_NOFS
into the GFP flags.

So are we sure it is broken and needs mending?