Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[David Hildenbrand]

On 06.06.24 03:34, Barry Song wrote:
> On Thu, Jun 6, 2024 at 2:42 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 05.06.24 16:08, Zi Yan wrote:
> > > On 5 Jun 2024, at 2:54, ran xiaokai wrote:
> > >
> > > > > On Tue, Jun 4, 2024 at 5:47?PM <xu.xin16@xxxxxxxxxx> wrote:
> > > > > > From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> > > > > >
> > > > > > When I did a large folios split test, a WARNING
> > > > > > "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
> > > > > > was triggered. But my test cases are only for anonmous folios.
> > > > > > while mapping_large_folio_support() is only reasonable for page
> > > > > > cache folios.
> > > > > >
> > > > > > In split_huge_page_to_list_to_order(), the folio passed to
> > > > > > mapping_large_folio_support() maybe anonmous folio. The
> > > > > > folio_test_anon() check is missing. So the split of the anonmous THP
> > > > > > is failed. This is also the same for shmem_mapping(). We'd better add
> > > > > > a check for both. But the shmem_mapping() in __split_huge_page() is
> > > > > > not involved, as for anonmous folios, the end parameter is set to -1, so
> > > > > > (head[i].index >= end) is always false. shmem_mapping() is not called.
> > > > > >
> > > > > > Using /sys/kernel/debug/split_huge_pages to verify this, with this
> > > > > > patch, large anon THP is successfully split and the warning is ceased.
> > > > > >
> > > > > > Signed-off-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> > > > > > Cc: xu xin <xu.xin16@xxxxxxxxxx>
> > > > > > Cc: Yang Yang <yang.yang29@xxxxxxxxxx>
> > > > > > ---
> > > > > >    mm/huge_memory.c | 38 ++++++++++++++++++++------------------
> > > > > >    1 file changed, 20 insertions(+), 18 deletions(-)
> > > > > >
> > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > > > index 317de2afd371..4c9c7e5ea20c 100644
> > > > > > --- a/mm/huge_memory.c
> > > > > > +++ b/mm/huge_memory.c
> > > > > > @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> > > > > >           if (new_order >= folio_order(folio))
> > > > > >                   return -EINVAL;
> > > > > >
> > > > > > -       /* Cannot split anonymous THP to order-1 */
> > > > > > -       if (new_order == 1 && folio_test_anon(folio)) {
> > > > > > -               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> > > > > > -               return -EINVAL;
> > > > > > -       }
> > > > > > -
> > > > > >           if (new_order) {
> > > > > >                   /* Only swapping a whole PMD-mapped folio is supported */
> > > > > >                   if (folio_test_swapcache(folio))
> > > > > >                           return -EINVAL;
> > > > > > -               /* Split shmem folio to non-zero order not supported */
> > > > > > -               if (shmem_mapping(folio->mapping)) {
> > > > > > -                       VM_WARN_ONCE(1,
> > > > > > -                               "Cannot split shmem folio to non-0 order");
> > > > > > -                       return -EINVAL;
> > > > > > -               }
> > > > > > -               /* No split if the file system does not support large folio */
> > > > > > -               if (!mapping_large_folio_support(folio->mapping)) {
> > > > > > -                       VM_WARN_ONCE(1,
> > > > > > -                               "Cannot split file folio to non-0 order");
> > > > > > -                       return -EINVAL;
> > > > > > +
> > > > > > +               if (folio_test_anon(folio)) {
> > > > > > +                       /* Cannot split anonymous THP to order-1 */
> > > > > > +                       if (new_order == 1) {
> > > > > > +                               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> > > > > > +                               return -EINVAL;
> > > > > > +                       }
> > > > > > +               } else {
> > > > > > +                       /* Split shmem folio to non-zero order not supported */
> > > > > > +                       if (shmem_mapping(folio->mapping)) {
> > > > > > +                               VM_WARN_ONCE(1,
> > > > > > +                                       "Cannot split shmem folio to non-0 order");
> > > > > > +                               return -EINVAL;
> > > > > > +                       }
> > > > > > +                       /* No split if the file system does not support large folio */
> > > > > > +                       if (!mapping_large_folio_support(folio->mapping)) {
> > > > > > +                               VM_WARN_ONCE(1,
> > > > > > +                                       "Cannot split file folio to non-0 order");
> > > > > > +                               return -EINVAL;
> > > > > > +                       }
> > > > >
> > > > > Am I missing something? if file system doesn't support large folio,
> > > > > how could the large folio start to exist from the first place while its
> > > > > mapping points to a file which doesn't support large folio?
> > > >
> > > > I think it is the CONFIG_READ_ONLY_THP_FOR_FS case.
> > > > khugepaged will try to collapse read-only file-backed pages to 2M THP.
> > >
> > > Can you add this information to the commit log in your next version?
> >
> > Can we also add that as a comment to that function, like "Note that we
> > might still
> > have THPs in such mappings due to CONFIG_READ_ONLY_THP_FOR_FS. But in
> > that case,
> > the mapping does not actually support large folios properly.
> > "Or sth. like that.
>
> +1. Otherwise, the code appears quite confusing.
> Would using "#ifdef" help to further clarify it?
>
> #ifdef CONFIG_READ_ONLY_THP_FOR_FS
>              if (!mapping_large_folio_support(folio->mapping)) {
>                            ....
>              }

if (IS_ENABLED()) ...

Agreed.

--
Cheers,

David / dhildenb