Re: [PATCH 2/5] kernel.h: Add non_block_start/end()

Jason Gunthorpe
2019-08-16T12:19:06Z

On Fri, Aug 16, 2019 at 10:10:29AM +0200, Michal Hocko wrote:
> On Thu 15-08-19 17:13:23, Jason Gunthorpe wrote:
> > On Thu, Aug 15, 2019 at 09:35:26PM +0200, Michal Hocko wrote:
> >
> > > > The last detail is I'm still unclear what a GFP flags a blockable
> > > > invalidate_range_start() should use. Is GFP_KERNEL OK?
> > >
> > > I hope I will not make this muddy again ;)
> > > invalidate_range_start in the blockable mode can use/depend on any sleepable
> > > allocation allowed in the context it is called from.
> >
> > 'in the context is is called from' is the magic phrase, as
> > invalidate_range_start is called while holding several different mm
> > related locks. I know at least write mmap_sem and i_mmap_rwsem
> > (write?)
> >
> > Can GFP_KERNEL be called while holding those locks?
>
> i_mmap_rwsem would be problematic because it is taken during the
> reclaim.

Okay.. So the fs_reclaim debugging does catch errors. Do you have any
reference for what a false positive looks like?

I would like to inject it into the notifier path as this is very
difficult for driver authors to discover and know about, but I'm
worried about your false positive remark.

I think I understand we can use only GFP_ATOMIC in the notifiers, but
we need a strategy to handle OOM to guarentee forward progress.

This is just more bugs to fix :(

Jason