Re: Unifying page table walkers

[David Hildenbrand <david@redhat.com>]

On 06.06.24 20:29, Matthew Wilcox wrote:
> One of the things we discussed at LSFMM was unifying the hugetlb and
> THP page table walkers.  I've been looking into it some more recently;
> I've found a problem and I think a solution.
> 
> The reason we have a separate hugetlb_entry from pmd_entry and pud_entry
> is that it has a different locking context.  It is called with the
> hugetlb_vma_lock held for read (nb: this is not the same as the vma
> lock; see walk_hugetlb_range()).  Why do we need this?  Because of page
> table sharing.
> 
> In a completely separate discussion, I was talking with Khalid about
> mshare() support for hugetlbfs, and I suggested that we permit hugetlbfs
> pages to be mapped by a VMA which does not have the VM_HUGETLB flag set.
> If we do that, the page tables would not be permitted to be shared with
> other users of that hugetlbfs file.  But we want to eliminate support
> for that anyway, so that's more of a feature than a bug.

I am not sure why hugetlb support in mshare would require that (we don't 
need partial mappings and all of that to support mshare+hugetlb).

The possible mshare directions I discussed with Khalid at LSF/MM would 
likely not need that. But I have no idea which mshare design you and 
Khalid are discussing right now. Maybe it would be a a good idea that 
the three of us meet to discuss that, if my feedback/opinion could be 
helpful.

> 
> Once we don't use the VM_HUGETLB flag on these VMAs, that opens the
> door to the other features we want, like mapping individual pages from
> a hugetlb folio.  And we can use the regular page table walkers for
> these VMAs.

Right, but to me that's a different, long-term project that mshare would 
maybe not have to rely on.

-- 
Cheers,

David / dhildenb