On 12/9/21 7:35 AM, Andrew Morton wrote:
On Wed, 8 Dec 2021 21:39:30 +0800 Li Xinhai <lixinhai.lxh@xxxxxxxxx> wrote:When BUG_ON check for THP migration entry, we need also consider !thp_migration_supported() case, becaouse it is invalid at the code context. Now, make this check been covered without adding extra cost. Because pmdval instead of *pmd is read, so just call pmd_migration_entry_wait(). Signed-off-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx> --- mm/gup.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 2c51e9748a6a..49f490bae030 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, if (!pmd_present(pmdval)) { if (likely(!(flags & FOLL_MIGRATION))) return no_page_table(vma, flags); - VM_BUG_ON(thp_migration_supported() && + VM_BUG_ON(!thp_migration_supported() ||I don't get this. thp_migration_supported() evaluates to a compile-time constant. What is the point in doing a runtime BUG based on a thing which is known at compile-time?
Yes, support thp migration is compile-time known. The exsiting VM_BUG_ON will be nothing if thp_migration_supported() equal to false, so pmdval other than migration entry will not be caught. Now, with the patch, if thp_migration_supported() equal to false, we will have VM_BUG_ON(1) so to catch above error case.
!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
