Re: [v2 PATCH 3/8] mm: khugepaged: skip DAX vma

[Yang Shi <shy828301@gmail.com>]

On Mon, Mar 21, 2022 at 5:04 AM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
>
> On Thu, Mar 17, 2022 at 04:48:22PM -0700, Yang Shi wrote:
> > The DAX vma may be seen by khugepaged when the mm has other khugepaged
> > suitable vmas.  So khugepaged may try to collapse THP for DAX vma, but
> > it will fail due to page sanity check, for example, page is not
> > on LRU.
> >
> > So it is not harmful, but it is definitely pointless to run khugepaged
> > against DAX vma, so skip it in early check.
>
> in fs/xfs/xfs_file.c:
> 1391 STATIC int
> 1392 xfs_file_mmap(
> 1393         struct file             *file,
> 1394         struct vm_area_struct   *vma)
> 1395 {
> 1396         struct inode            *inode = file_inode(file);
> 1397         struct xfs_buftarg      *target = xfs_inode_buftarg(XFS_I(inode));
> 1398
> 1399         /*
> 1400          * We don't support synchronous mappings for non-DAX files and
> 1401          * for DAX files if underneath dax_device is not synchronous.
> 1402          */
> 1403         if (!daxdev_mapping_supported(vma, target->bt_daxdev))
> 1404                 return -EOPNOTSUPP;
> 1405
> 1406         file_accessed(file);
> 1407         vma->vm_ops = &xfs_file_vm_ops;
> 1408         if (IS_DAX(inode))
> 1409                 vma->vm_flags |= VM_HUGEPAGE;
>
> Are xfs and other filesystems setting VM_HUGEPAGE flag even if it can
> never be collapsed?

DAX is available or intended to use on some special devices, for
example, persistent memory. Collapsing huge pages on such devices is
not the intended usecase of khugepaged.

>
> 1410         return 0;
> 1411 }
>
>
> > Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/khugepaged.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 82c71c6da9ce..a0e4fa33660e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -448,6 +448,10 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
> >       if (vm_flags & VM_NO_KHUGEPAGED)
> >               return false;
> >
> > +     /* Don't run khugepaged against DAX vma */
> > +     if (vma_is_dax(vma))
> > +             return false;
> > +
> >       if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
> >                               vma->vm_pgoff, HPAGE_PMD_NR))
> >               return false;
> > --
> > 2.26.3
> >
> >
>
> --
> Thank you, You are awesome!
> Hyeonggon :-)