From: Li Xinhai <lixinhai.lxh@xxxxxxxxx> Subject: 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@xxxxxxxxx Signed-off-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx> Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx> Cc: Zi Yan <ziy@xxxxxxxxxx> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/gup.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) --- a/mm/gup.c~mm-gupc-stricter-check-on-thp-migration-entry-during-follow_pmd_mask +++ a/mm/gup.c @@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(stru } 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 _
