mm: khugepaged: skip DAX vma

The DAX vma may be seen by khugepaged when the mm has other khugepaged
suitable vmas.  So khugepaged may try to collapse THP for DAX vma, but it
will fail due to page sanity check, for example, page is not on LRU.

So it is not harmful, but it is definitely pointless to run khugepaged
against DAX vma, so skip it in early check.

Link: https://lkml.kernel.org/r/20220510203222.24246-4-shy828301@gmail.comSigned-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Vlastmil Babka <vbabka@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/khugepaged.c

@@ -447,6 +447,10 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
 	if (vm_flags & VM_NO_KHUGEPAGED)
 		return false;
 
+	/* Don't run khugepaged against DAX vma */
+	if (vma_is_dax(vma))
+		return false;
+
 	if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
 				vma->vm_pgoff, HPAGE_PMD_NR))
 		return false;