Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Thu, Jan 14, 2021 at 7:27 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 1/13/21 1:20 AM, Oscar Salvador wrote:
> > On Tue, Jan 12, 2021 at 07:33:33PM +0800, Muchun Song wrote:
> >>> It seems a bit odd to only pass "start" for the BUG_ON.
> >>> Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
> >>>
> >>> I wonder if adding a ".remap_start_addr" would make more sense.
> >>> And adding it here with the vmemmap_remap_walk init.
> >>
> >> How about introducing a new function which aims to get the reuse
> >> page? In this case, we can drop the BUG_ON() and "addr += PAGE_SIZE"
> >> which is in vmemmap_pte_range. The vmemmap_remap_range only
> >> does the remapping.
> >
> > How would that look?
> > It might be good, dunno, but the point is, we should try to make the rules as
> > simple as possible, dropping weird assumptions.
> >
> > Callers of vmemmap_remap_free should know three things:
> >
> > - Range to be remapped
> > - Addr to remap to
> > - Current implemantion needs addr to be remap to to be part of the complete
> >   range
> >
> > right?
>
> And, current implementation needs must have remap addr be the first in the
> complete range.  This is just because of the way the page tables are walked
> for remapping.  The remap/reuse page must be found first so that the following
> pages can be remapped to it.

You are right.

>
> That implementation seems to be the 'most efficient' for hugetlb pages where
> we want vmemmap pages n+3 and beyond mapped to n+2.
>
> In a more general purpose vmemmap_remap_free implementation, the reuse/remap
> address would not necessarily need to be related to the range.  However, this
> would require a separate page table walk/validation for the reuse address
> independent of the range.  This may be what Muchun was proposing for 'a new
> function which aims to get the reuse page'.

Agree.


>
> IMO, the decision on how to implement depends on the intended use case.
> - If this is going to be hugetlb only (or perhaps generic huge page only)
>   functionality, then I am OK with an efficient implementation that has
>   some restrictions.
> - If we see this being used for more general purpose remapping, then we
>   should go with a more general purpose implementation.

I think this approach may be only suitable for generic huge page only.
So we can implement it only for huge page.

Hi Oscar,

What's your opinion about this?

>
> Again, just my opinion.
> --
> Mike Kravetz