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

[Michal Hocko]

On Fri 16-08-19 09:19:06, Jason Gunthorpe wrote:
> 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.

I do not think fs_reclaim is the udnerlying mechanism to catch this
deadlock. It is a simple AA deadlock. You take i_mmap_rwsem and then
go down the allocation path, direct reclaim and take the lock again.
Nothing really surprising. fs_reclaim is really to catch GFP_NOFS
context calling into a less restricted (e.g. GFP_KERNEL allocation
context).

> Do you have any
> reference for what a false positive looks like? 

I believe I have given some examples when introducing __GFP_NOLOCKDEP.
 
> 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.

Your example is from the notifier registration IIUC. Can you
pre-allocate before taking locks? Could you point me to some examples
when the allocation is necessary in the range notifier callback?
-- 
Michal Hocko
SUSE Labs