mm/huge_memory.c: fix potential NULL pointer dereference
mm/huge_memory.c: fix potential NULL pointer dereference

In collapse_huge_page() there is a race window between releasing the
mmap_sem read lock and taking the mmap_sem write lock, so find_vma() may
return NULL.  So check the return value to avoid NULL pointer dereference.

collapse_huge_page
	khugepaged_alloc_page
		up_read(&mm->mmap_sem)
	down_write(&mm->mmap_sem)
	vma = find_vma(mm, address)

Signed-off-by: Libin <907cb18f632f56970a1dc2ab34acaa162917635f@huawei.com>
Acked-by: Kirill A. Shutemov <eec72676ad9a62f2727dfed6d270d98c0d02adec@linux.intel.com>
Reviewed-by: Wanpeng Li <86d530c4a0b810dc887df1e4e084ed44c9b7acac@linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <829500b200af6c2a0eca4dfd5c388e6b8fbe8dc6@suse.cz>
Cc: <4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org> # v3.0+
Signed-off-by: Andrew Morton <5c1e68b099950c134891f0b6e179498a8ebe9cf9@linux-foundation.org>
Signed-off-by: Linus Torvalds <69652caca27c8b940640ad396ab71f93cacec34f@linux-foundation.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs