[PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

[Li Xinhai <lixinhai.lxh@gmail.com>]

When BUG_ON check for THP migration entry, the exsiting code only check
thp_migration_supported case, but not for !thp_migration_supported case.
If !thp_migration_supported() and !pmd_present(), the original code may
dead loop in theory. To make the BUG_ON check consistent, we need catch
both cases.

Move the BUG_ON check one step eariler, because if the bug happen we
should know it instead of depend on FOLL_MIGRATION been used by caller.

Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
check, the existing code don't help to avoid useless locking within
pmd_migration_entry_wait(), so remove that check.

Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
---
V2->V3:
mention about the dead loop in commit message.

V1->V2:
Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
for it. 


 mm/gup.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 2c51e9748a6a..94d0e586ca0b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 	}
 retry:
 	if (!pmd_present(pmdval)) {
+		/*
+		 * Should never reach here, if thp migration is not supported;
+		 * Otherwise, it must be a thp miration entry.
+		 */
+		VM_BUG_ON(!thp_migration_supported() ||
+				  !is_pmd_migration_entry(pmdval));
+
 		if (likely(!(flags & FOLL_MIGRATION)))
 			return no_page_table(vma, flags);
-		VM_BUG_ON(thp_migration_supported() &&
-				  !is_pmd_migration_entry(pmdval));
-		if (is_pmd_migration_entry(pmdval))
-			pmd_migration_entry_wait(mm, pmd);
+
+		pmd_migration_entry_wait(mm, pmd);
 		pmdval = READ_ONCE(*pmd);
 		/*
 		 * MADV_DONTNEED may convert the pmd to null because
-- 
2.27.0