Re: [PATCH] fuse: when copying a folio delay the mark dirty until the end

[Joanne Koong <joannelkoong@gmail.com>]

On Mon, Mar 16, 2026 at 8:16 AM Horst Birthelmer <horst@birthelmer.com> wrote:
>
> From: Horst Birthelmer <hbirthelmer@ddn.com>
>
> Doing set_page_dirty_lock() for every page is inefficient
> for large folios.
> When copying a folio (and with large folios enabled,
> this can be many pages) we can delay the marking dirty
> and flush_dcache_page() until the whole folio is handled
> and do it once per folio instead of once per page.
>
> Signed-off-by: Horst Birthelmer <hbirthelmer@ddn.com>
> ---
> Currently when doing a folio copy
> flush_dcache_page(cs->pg) and set_page_dirty_lock(cs->pg)
> are called for every page.
>
> We can do this at the end for the whole folio.

Hi Horst,

I think these are two different entities. cs->pg is the page that
corresponds to the userspace buffer / pipe while the (large) folio
corresponds to the pages in the page cache. flush_dcache_folio(folio)
and flush_dcache_page(cs->pg) are not interchangeable (I don't think
it's likely either that the pages backing the userspace buffer/pipe
are large folios).

Thanks,
Joanne

> ---
>  fs/fuse/dev.c        | 9 +++++++--
>  fs/fuse/fuse_dev_i.h | 1 +
>  2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 0b0241f47170d4640f0b8f3cae8be1f78944a456..ae96a48f898e883b4e96147f3b27398261c5e844 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -855,7 +855,7 @@ void fuse_copy_finish(struct fuse_copy_state *cs)
>                         buf->len = PAGE_SIZE - cs->len;
>                 cs->currbuf = NULL;
>         } else if (cs->pg) {
> -               if (cs->write) {
> +               if (cs->write && !cs->copy_folio) {
>                         flush_dcache_page(cs->pg);
>                         set_page_dirty_lock(cs->pg);
>                 }
> @@ -1126,6 +1126,7 @@ static int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
>                         folio_zero_range(folio, 0, size);
>         }
>
> +       cs->copy_folio = true;
>         while (count) {
>                 if (cs->write && cs->pipebufs && folio) {
>                         /*
> @@ -1167,8 +1168,12 @@ static int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
>                 } else
>                         offset += fuse_copy_do(cs, NULL, &count);
>         }
> -       if (folio && !cs->write)
> +       if (folio) {
>                 flush_dcache_folio(folio);
> +               if (cs->write)
> +                       folio_mark_dirty_lock(folio);
> +       }
> +       cs->copy_folio = false;
>         return 0;
>  }
>
> diff --git a/fs/fuse/fuse_dev_i.h b/fs/fuse/fuse_dev_i.h
> index 134bf44aff0d39ae8d5d47cf1518efcf2f1cfc23..4a433d902266d573ad1c19adbdd573440e2a77b2 100644
> --- a/fs/fuse/fuse_dev_i.h
> +++ b/fs/fuse/fuse_dev_i.h
> @@ -33,6 +33,7 @@ struct fuse_copy_state {
>         unsigned int offset;
>         bool write:1;
>         bool move_folios:1;
> +       bool copy_folio:1;
>         bool is_uring:1;
>         struct {
>                 unsigned int copied_sz; /* copied size into the user buffer */
>
> ---
> base-commit: f338e77383789c0cae23ca3d48adcc5e9e137e3c
> change-id: 20260316-mark-dirty-per-folio-be87b6b4bf56
>
> Best regards,
> --
> Horst Birthelmer <hbirthelmer@ddn.com>
>