Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

[Peter Zijlstra]

On Mon, May 13, 2019 at 10:36:06AM +0200, Peter Zijlstra wrote:
> On Thu, May 09, 2019 at 09:21:35PM +0000, Nadav Amit wrote:
> > It may be possible to avoid false-positive nesting indications (when the
> > flushes do not overlap) by creating a new struct mmu_gather_pending, with
> > something like:
> > 
> >   struct mmu_gather_pending {
> >  	u64 start;
> > 	u64 end;
> > 	struct mmu_gather_pending *next;
> >   }
> > 
> > tlb_finish_mmu() would then iterate over the mm->mmu_gather_pending
> > (pointing to the linked list) and find whether there is any overlap. This
> > would still require synchronization (acquiring a lock when allocating and
> > deallocating or something fancier).
> 
> We have an interval_tree for this, and yes, that's how far I got :/
> 
> The other thing I was thinking of is trying to detect overlap through
> the page-tables themselves, but we have a distinct lack of storage
> there.

We might just use some state in the pmd, there's still 2 _pt_pad_[12] in
struct page to 'use'. So we could come up with some tlb generation
scheme that would detect conflict.