Re: [PATCH 6/6] shmem: add large folios support to the write path

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Sep 15, 2023 at 2:51 AM Daniel Gomez <da.gomez@samsung.com> wrote:
>
> Add large folio support for shmem write path matching the same high
> order preference mechanism used for iomap buffered IO path as used in
> __filemap_get_folio().
>
> Use the __folio_get_max_order to get a hint for the order of the folio
> based on file size which takes care of the mapping requirements.
>
> Swap does not support high order folios for now, so make it order 0 in
> case swap is enabled.

I didn't take a close look at the series, but I am not sure I
understand the rationale here. Reclaim will split high order shmem
folios anyway, right?

It seems like we only enable high order folios if the "noswap" mount
option is used, which is fairly recent. I doubt it is widely used.

>
> Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
> ---
>  mm/shmem.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index adff74751065..26ca555b1669 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1683,13 +1683,19 @@ static struct folio *shmem_alloc_folio(gfp_t gfp,
>  }
>
>  static struct folio *shmem_alloc_and_acct_folio(gfp_t gfp, struct inode *inode,
> -               pgoff_t index, bool huge, unsigned int *order)
> +               pgoff_t index, bool huge, unsigned int *order,
> +               struct shmem_sb_info *sbinfo)
>  {
>         struct shmem_inode_info *info = SHMEM_I(inode);
>         struct folio *folio;
>         int nr;
>         int err;
>
> +       if (!sbinfo->noswap)
> +               *order = 0;
> +       else
> +               *order = (*order == 1) ? 0 : *order;
> +
>         if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
>                 huge = false;
>         nr = huge ? HPAGE_PMD_NR : 1U << *order;
> @@ -2032,6 +2038,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>                 return 0;
>         }
>
> +       order = mapping_size_order(inode->i_mapping, index, len);
> +
>         if (!shmem_is_huge(inode, index, false,
>                            vma ? vma->vm_mm : NULL, vma ? vma->vm_flags : 0))
>                 goto alloc_nohuge;
> @@ -2039,11 +2047,11 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>         huge_gfp = vma_thp_gfp_mask(vma);
>         huge_gfp = limit_gfp_mask(huge_gfp, gfp);
>         folio = shmem_alloc_and_acct_folio(huge_gfp, inode, index, true,
> -                                          &order);
> +                                          &order, sbinfo);
>         if (IS_ERR(folio)) {
>  alloc_nohuge:
>                 folio = shmem_alloc_and_acct_folio(gfp, inode, index, false,
> -                                                  &order);
> +                                                  &order, sbinfo);
>         }
>         if (IS_ERR(folio)) {
>                 int retry = 5;
> @@ -2147,6 +2155,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>         if (folio_test_large(folio)) {
>                 folio_unlock(folio);
>                 folio_put(folio);
> +               if (order > 0)
> +                       order--;
>                 goto alloc_nohuge;
>         }
>  unlock:
> --
> 2.39.2
>