linux-ext4 - Re: [PATCH 2/3] mm: Add folio_fill

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Date:   Thu, 9 Nov 2023 22:50:45 +0100
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4@...r.kernel.org, gfs2@...ts.linux.dev,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        "Darrick J . Wong" <djwong@...nel.org>,
        linux-erofs@...ts.ozlabs.org, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: [PATCH 2/3] mm: Add folio_fill_tail() and use it in iomap

On Tue, Nov 7, 2023 at 10:27 PM Matthew Wilcox (Oracle)
<willy@...radead.org> wrote:
> The iomap code was limited to PAGE_SIZE bytes; generalise it to cover
> an arbitrary-sized folio, and move it to be a common helper.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  fs/iomap/buffered-io.c  | 14 ++------------
>  include/linux/highmem.h | 38 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 40 insertions(+), 12 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f72df2babe56..093c4515b22a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -305,28 +305,18 @@ static int iomap_read_inline_data(const struct iomap_iter *iter,
>  {
>         const struct iomap *iomap = iomap_iter_srcmap(iter);
>         size_t size = i_size_read(iter->inode) - iomap->offset;
> -       size_t poff = offset_in_page(iomap->offset);
>         size_t offset = offset_in_folio(folio, iomap->offset);
> -       void *addr;
>
>         if (folio_test_uptodate(folio))
>                 return 0;
>
> -       if (WARN_ON_ONCE(size > PAGE_SIZE - poff))
> -               return -EIO;
> -       if (WARN_ON_ONCE(size > PAGE_SIZE -
> -                        offset_in_page(iomap->inline_data)))
> -               return -EIO;
>         if (WARN_ON_ONCE(size > iomap->length))
>                 return -EIO;
>         if (offset > 0)
>                 ifs_alloc(iter->inode, folio, iter->flags);
>
> -       addr = kmap_local_folio(folio, offset);
> -       memcpy(addr, iomap->inline_data, size);
> -       memset(addr + size, 0, PAGE_SIZE - poff - size);
> -       kunmap_local(addr);
> -       iomap_set_range_uptodate(folio, offset, PAGE_SIZE - poff);
> +       folio_fill_tail(folio, offset, iomap->inline_data, size);
> +       iomap_set_range_uptodate(folio, offset, folio_size(folio) - offset);
>         return 0;
>  }
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 1b81416196dd..0fbb60ffefc9 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -521,6 +521,44 @@ static inline __must_check void *folio_zero_tail(struct folio *folio,
>         return kaddr;
>  }
>
> +/**
> + * folio_fill_tail - Copy some data to a folio and pad with zeroes.
> + * @folio: The destination folio.
> + * @offset: The offset into @folio at which to start copying.
> + * @from: The data to copy.
> + * @len: How many bytes of data to copy.
> + *
> + * This function is most useful for filesystems which support inline data.
> + * When they want to copy data from the inode into the page cache, this
> + * function does everything for them.  It supports large folios even on
> + * HIGHMEM configurations.
> + */
> +static inline void folio_fill_tail(struct folio *folio, size_t offset,
> +               const char *from, size_t len)
> +{
> +       char *to = kmap_local_folio(folio, offset);
> +
> +       VM_BUG_ON(offset + len > folio_size(folio));
> +
> +       if (folio_test_highmem(folio)) {
> +               size_t max = PAGE_SIZE - offset_in_page(offset);
> +
> +               while (len > max) {
> +                       memcpy(to, from, max);
> +                       kunmap_local(to);
> +                       len -= max;
> +                       from += max;
> +                       offset += max;
> +                       max = PAGE_SIZE;
> +                       to = kmap_local_folio(folio, offset);
> +               }
> +       }
> +
> +       memcpy(to, from, len);
> +       to = folio_zero_tail(folio, offset, to);

This needs to be:

to = folio_zero_tail(folio, offset  + len, to + len);

> +       kunmap_local(to);
> +}
> +
>  /**
>   * memcpy_from_file_folio - Copy some bytes from a file folio.
>   * @to: The destination buffer.
> --
> 2.42.0
>

Reviewed-by: Andreas Gruenbacher <agruenba@...hat.com>

Thanks,
Andreas