Re: [PATCH] hugetlbfs: fix kernel BUG at fs/hugetlbfs/inode.c:444!

[Naoya Horiguchi]

On Mon, Nov 05, 2018 at 01:23:15PM -0800, Mike Kravetz wrote:
> This bug has been experienced several times by Oracle DB team.
> The BUG is in the routine remove_inode_hugepages() as follows:
> 	/*
> 	 * If page is mapped, it was faulted in after being
> 	 * unmapped in caller.  Unmap (again) now after taking
> 	 * the fault mutex.  The mutex will prevent faults
> 	 * until we finish removing the page.
> 	 *
> 	 * This race can only happen in the hole punch case.
> 	 * Getting here in a truncate operation is a bug.
> 	 */
> 	if (unlikely(page_mapped(page))) {
> 		BUG_ON(truncate_op);
> 
> In this case, the elevated map count is not the result of a race.
> Rather it was incorrectly incremented as the result of a bug in the
> huge pmd sharing code.  Consider the following:
> - Process A maps a hugetlbfs file of sufficient size and alignment
>   (PUD_SIZE) that a pmd page could be shared.
> - Process B maps the same hugetlbfs file with the same size and alignment
>   such that a pmd page is shared.
> - Process B then calls mprotect() to change protections for the mapping
>   with the shared pmd.  As a result, the pmd is 'unshared'.
> - Process B then calls mprotect() again to chage protections for the
>   mapping back to their original value.  pmd remains unshared.
> - Process B then forks and process C is created.  During the fork process,
>   we do dup_mm -> dup_mmap -> copy_page_range to copy page tables.  Copying
>   page tables for hugetlb mappings is done in the routine
>   copy_hugetlb_page_range.
> 
> In copy_hugetlb_page_range(), the destination pte is obtained by:
> 	dst_pte = huge_pte_alloc(dst, addr, sz);
> If pmd sharing is possible, the returned pointer will be to a pte in
> an existing page table.  In the situation above, process C could share
> with either process A or process B.  Since process A is first in the
> list, the returned pte is a pointer to a pte in process A's page table.
> 
> However, the following check for pmd sharing is in copy_hugetlb_page_range.
> 	/* If the pagetables are shared don't copy or take references */
> 	if (dst_pte == src_pte)
> 		continue;
> 
> Since process C is sharing with process A instead of process B, the above
> test fails.  The code in copy_hugetlb_page_range which follows assumes
> dst_pte points to a huge_pte_none pte.  It copies the pte entry from
> src_pte to dst_pte and increments this map count of the associated page.
> This is how we end up with an elevated map count.
> 
> To solve, check the dst_pte entry for huge_pte_none.  If !none, this
> implies PMD sharing so do not copy.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>