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 9:24 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> On 6 Jun 2024, at 14:00, Barry Song wrote:
>
> > 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
>
> That is why we have the warning below to catch this undesired
> case.
>
> > if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) is correct.
>
> When it is set, khugepaged can create a large pagecache folio
> on a filesystem without large folio support and the warning
> will be triggered once the created large pagecache folio
> is split. That is not what we want.

yes. This is exactly why we need if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS))
but not if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) .

because if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)), folio is definitely
pointing to a file system supporting large folio. otherwise, it is a bug.

>
> >
> > 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
>
>
> Best Regards,
> Yan, Zi