Re: [PATCH v2 6/9] mm: free user PTE page table pages

From: Jason Gunthorpe
Date: Wed Sep 01 2021 - 13:55:53 EST

On Wed, Sep 01, 2021 at 07:49:23PM +0200, David Hildenbrand wrote:
> On 01.09.21 19:10, Jason Gunthorpe wrote:
> > On Wed, Sep 01, 2021 at 06:19:03PM +0200, David Hildenbrand wrote:
> >
> > > > I wouldn't think it works everywhere, bit it works in a lot of places,
> > > > and it is a heck of a lot better than what is proposed here. I'd
> > > > rather see the places that can use it be moved, and the few places
> > > > that can't be opencoded.
> > >
> > > Well, I used ptep_get_map_lock() and friends. But hacking directly into
> > > ptep_map_lock() and friends wasn't possible due to all the corner cases.
> >
> > Sure, I'm not surprised you can't get every single case, but that just
> > suggest we need two API families, today's to support the special cases
> > and a different one for the other regular simple cases.
> >
> > A new function family pte_try_map/_locked() and paired unmap that can
> > internally do the recounting and THP trickery and convert the easy
> > callsites.
> >
> > Very rough counting suggest at least half of the pte_offset_map_lock()
> > call sites can trivially use the simpler API.
> >
> > The other cases can stay as is and get open coded refcounts, or maybe
> > someone will have a better idea once they are more clearly identified.
> >
> > But I don't think we should take a performance hit of additional
> > atomics in cases like GUP where this is trivially delt with by using a
> > better API.
> Right, but as I said in the cover letter, we can happily optimize once we
> have the basic infrastructure in place and properly reviewed. Getting rid of
> some unnecessary atomics by introducing additional fancy helpers falls under
> that category.

I'm not sure I agree given how big and wide this patch series is. It
would be easier to review if it was touching less places. The helpers
are not fancy, it is a logical re-arrangement of existing code that
shrinks the LOC of this series and makes it more reviewable.

Or stated another way, a niche feature like this try much harder not
to add more complexity everywhere.