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

[ran xiaokai]

> 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.