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

[Peter Xu]

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

So it's not the same logic as how the RCU lock worked, but it's actually
better (even though with higher overhead) because vma unlink happens before
free_pgd_range(), so the pgtable locks are not freed yet (unlike RCU).

Thanks,

-- 
Peter Xu