mm: migrate: remove PageHead() check for HugeTLB in add_page_for_migration()

There is some different between hugeTLB and THP behave when passed the
address of a tail page, for THP, it will migrate the entire THP page, but
for HugeTLB, it will return -EACCES, or -ENOENT before commit e66f17ff
("mm/hugetlb: take page table lock in follow_huge_pmd()"),

  -EACCES The page is mapped by multiple processes and can be moved
	  only if MPOL_MF_MOVE_ALL is specified.
  -ENOENT The page is not present.

But when check manual[1], both of the two errnos are not suitable, it is
better to keep the same behave between hugetlb and THP when passed the
address of a tail page, so let's just remove the PageHead() check for
HugeTLB.

[1] https://man7.org/linux/man-pages/man2/move_pages.2.html

Link: https://lkml.kernel.org/r/20230913095131.2426871-8-wangkefeng.wang@huawei.comSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/migrate.c

@@ -2096,10 +2096,8 @@ static int add_page_for_migration(struct mm_struct *mm, const void __user *p,
 		goto out_putfolio;
 
 	if (folio_test_hugetlb(folio)) {
-		if (PageHead(page)) {
 		isolated = isolate_hugetlb(folio, pagelist);
 		err = isolated ? 1 : -EBUSY;
-		}
 	} else {
 		isolated = folio_isolate_lru(folio);
 		if (!isolated) {