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.
Signed-off-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
---
V3->V4:
Fix typos
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..1b500ca2a1b8 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 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
--
2.27.0
