Re: [PATCH v2 0/9] Free user PTE page table pages

[Qi Zheng]

On 9/16/21 4:30 PM, David Hildenbrand wrote:
> On 16.09.21 07:32, Qi Zheng wrote:
> > On 9/15/21 10:59 PM, Jason Gunthorpe wrote:
> > > On Wed, Sep 15, 2021 at 10:52:40PM +0800, Qi Zheng wrote:
> > > > I am going to split this patch series as follows:
> > > >
> > > > 1. Introduce the new dummy APIs, which is an empty implementation.
> > > >      But I will explain its semantics.
> > > > 2. Merge #6, #7 and #8, and call these dummy APIs in any necessary
> > > >      location, and split some special cases into single patches, 
> > > > such as
> > > >      pagefault and gup, etc. So that we can explain in more detail the
> > > >      concurrency in these cases. For example, we don't need to hold any
> > > >      pte_refcount in the fast path in gup on the x86_64 platform. 
> > > > Because
> > > >      the PTE page can't be freed after the local CPU interrupt is 
> > > > closed
> > > >      in the fast path in gup.
> > > > 3. Introduce CONFIG_FREE_USER_PTE and implement these empty dummy APIs.
> > > > 4. Add a description document.
> > > >
> > > > And I try to add a function that combines pte_offset_map() and
> > > > pte_try_get(). Maybe the func name is pte_try_map() recommended by
> > > > Jason, or keep the pte_offset_map() unchanged?
> > >
> > > It is part of the transformation, add a
> > > pte_try_map()/pte_undo_try_map() and replace all the pte_offset_map()
> > > callsites that can use the new API with it. The idea was that try_map
> > > would incorporate the pmd_trans_unstable/etc mess so searching for
> > > trans_unstable is a good place to start finding candidates. Some are
> > > simple, some are tricky.
> >
> > Yes, I will search pte_offset_map()/pmd_trans_unstable/etc, and then
> > analyze the specific situation.
>
> Maybe propose the new API first, before doing the actual implementation. 
> Might safe you from doing some additional back-and-forth
> work eventually.

OK, I prepare to propose all the dummy APIs in the step 1.

Thanks,
Qi

>