Re: [PATCH linux-next v2] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[Barry Song]

On Fri, Jun 7, 2024 at 2:35 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> +Matthew
>
> For mapping_large_folio_support() changes.
>
> On 6 Jun 2024, at 2:42, 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 the 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.
> >
> > Also add a VM_WARN_ON_ONCE() in mapping_large_folio_support()
> > for anon mapping, So we can detect the wrong use more easily.
> >
> > THP folios maybe exist in the pagecache even the file system doesn't
> > support large folio, it is because when CONFIG_TRANSPARENT_HUGEPAGE
> > is enabled, khugepaged will try to collapse read-only file-backed pages
> > to THP. But the mapping does not actually support multi order
> > large folios properly.
> >
> > 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>
> > ---
> >  include/linux/pagemap.h |  4 ++++
> >  mm/huge_memory.c        | 27 ++++++++++++++++-----------
> >  2 files changed, 20 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index ee633712bba0..59f1df0cde5a 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -381,6 +381,10 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
> >   */
> >  static inline bool mapping_large_folio_support(struct address_space *mapping)
> >  {
> > +     /* AS_LARGE_FOLIO_SUPPORT is only reasonable for pagecache folios */
> > +     VM_WARN_ONCE((unsigned long)mapping & PAGE_MAPPING_ANON,
> > +                     "Anonymous mapping always supports large folio");
> > +
> >       return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> >               test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
> >  }
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 317de2afd371..62d57270b08e 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3009,30 +3009,35 @@ 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))
> > +     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 if (new_order) {
> >               /* 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)) {
> > +             /* No split if the file system does not support large folio.
> > +              * 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.
> > +              */
> > +             if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> > +                     !mapping_large_folio_support(folio->mapping)) {
>
> Shouldn’t this be
>
> if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>         !mapping_large_folio_support(folio->mapping)) {
>
> ?
>
> When CONFIG_READ_ONLY_THP_FOR_FS is not set, we need to check
> mapping_large_folio_support(), otherwise we do not.

while CONFIG_READ_ONLY_THP_FOR_FS is not set, that is no way
a large folio can be mapped to a filesystem which doesn't support
large folio mapping. i think
if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) is correct.

The below means a BUG which has never a chance to happen if it
is true.

!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
        !mapping_large_folio_support(folio->mapping));

>
> >                       VM_WARN_ONCE(1,
> >                               "Cannot split file folio to non-0 order");
> >                       return -EINVAL;
> >               }
> >       }
> >
> > +     /* Only swapping a whole PMD-mapped folio is supported */
> > +     if (folio_test_swapcache(folio) && new_order)
> > +             return -EINVAL;
> >
> >       is_hzp = is_huge_zero_folio(folio);
> >       if (is_hzp) {
> > --
> > 2.15.2
>
>
> Best Regards,
> Yan, Zi

Thanks
Barry