On Tue, Jun 21, 2016 at 10:05:56PM +0800, zhongjiang wrote:

From: zhong jiang <zhongjiang@huawei.com>

with great pressure, I run some test cases. As a result, I found
that the THP is not freed, it is detected by check_mm().

BUG: Bad rss-counter state mm:ffff8827edb70000 idx:1 val:512

Consider the following race :

	CPU0                               CPU1
  __handle_mm_fault()
        wp_huge_pmd()
   	    do_huge_pmd_wp_page()
		pmdp_huge_clear_flush_notify()
                (pmd_none = true)
					exit_mmap()
					   unmap_vmas()
					     zap_pmd_range()
						pmd_none_or_trans_huge_or_clear_bad()
						   (result in memory leak)
                set_pmd_at()

because of CPU0 have allocated huge page before pmdp_huge_clear_notify,
and it make the pmd entry to be null. Therefore, The memory leak can occur.

The patch fix the scenario that the pmd entry can lead to be null.

I don't think the scenario is possible.

exit_mmap() called when all mm users have gone, so no parallel threads
exist.