Re: [PATCH 2/3] xfs: add kmem_alloc_io()

Peter Zijlstra
2019-08-22T08:51:40Z

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.