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

[Barry Song]

On Thu, Jun 6, 2024 at 9:42 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 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 */

This is simply what the code is indicating. Shouldn't we phrase
it differently to explain "why" but not "how"? for example, anon
order-1 mTHP is not supported?

Otherwise, it looks good to me.

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

> +               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)) {
>                         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
>