Re: [PATCH] hugetlbfs: remove call to huge_pte_alloc without i_mmap_rwsem

[Mike Kravetz]

On 8/3/20 3:52 PM, Matthew Wilcox wrote:
> On Mon, Aug 03, 2020 at 03:43:35PM -0700, Mike Kravetz wrote:
>> Commit c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
>> synchronization") requires callers of huge_pte_alloc to hold i_mmap_rwsem
>> in at least read mode.  This is because the explicit locking in
>> huge_pmd_share (called by huge_pte_alloc) was removed.  When restructuring
>> the code, the call to huge_pte_alloc in the else block at the beginning
>> of hugetlb_fault was missed.
> 
> Should we have a call to mmap_assert_locked() in huge_pte_alloc(),
> at least the generic one?

That is the wrong semaphore.

However, I was not aware of the checks for a semaphore being held as is
done in rwsem_is_locked().  That would have caught this when the original
code was changed.  Thanks for pointing this out.

Let me update the patch and add checks to huge_pmd_share().
-- 
Mike Kravetz