mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

When BUG_ON check for THP migration entry, the existing 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 earlier, 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.

Link: https://lkml.kernel.org/r/20211217062559.737063-1-lixinhai.lxh@gmail.com
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index be2a41feec7d7b85c558d3d9b7e19a7ea8b272c3..f0af462ac1e2b6695dda14c085a856b498188e4b 100644 (file)
--- 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 migration 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