Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage

Mike Kravetz
2022-12-05T21:51:47Z

On 11/30/22 10:58, Peter Xu wrote:
> Hi, Mike,
>
> On Tue, Nov 29, 2022 at 08:55:21PM -0800, Mike Kravetz wrote:
> > > + * (2) For shared mappings: pmd unsharing is possible (so the PUD-ranged
> > > + * pgtable page can go away from under us! It can be done by a pmd
> > > + * unshare with a follow up munmap() on the other process), then we
> > > + * need either:
> > > + *
> > > + * (2.1) hugetlb vma lock read or write held, to make sure pmd unshare
> > > + * won't happen upon the range (it also makes sure the pte_t we
> > > + * read is the right and stable one), or,
> > > + *
> > > + * (2.2) hugetlb mapping i_mmap_rwsem lock held read or write, to make
> > > + * sure even if unshare happened the racy unmap() will wait until
> > > + * i_mmap_rwsem is released.
> >
> > Is that 100% correct? IIUC, the page tables will be released via the
> > call to tlb_finish_mmu(). In most cases, the tlb_finish_mmu() call is
> > performed when holding i_mmap_rwsem. However, in the final teardown of
> > a hugetlb vma via __unmap_hugepage_range_final, the tlb_finish_mmu call
> > is done outside the i_mmap_rwsem lock. In this case, I think we are
> > still safe because nobody else should be walking the page table.
> >
> > I really like the documentation. However, if i_mmap_rwsem is not 100%
> > safe I would prefer not to document it here. I don't think anyone
> > relies on this do they?
>
> I think i_mmap_rwsem is 100% safe.
>
> It's not in tlb_finish_mmu(), but when freeing the pgtables we need to
> unlink current vma from the vma list first:
>
> free_pgtables
> unlink_file_vma
> i_mmap_lock_write
> tlb_finish_mmu

Thanks!

Sorry, I was thinking about page freeing not page table freeing.

Agree that is 100% safe.
--
Mike Kravetz