[PATCH 1/3] mm: Add folio_zero_tail() and use it in ext4

[Andreas Gruenbacher]

Hi Willy,

On Tue, Nov 7, 2023 at 10:27 PM Matthew Wilcox (Oracle)
<willy at infradead.org> wrote:
> Instead of unmapping the folio after copying the data to it, then mapping
> it again to zero the tail, provide folio_zero_tail() to zero the tail
> of an already-mapped folio.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
> ---
>  fs/ext4/inline.c        |  3 +--
>  include/linux/highmem.h | 38 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> index 9a84a5f9fef4..d5bd1e3a5d36 100644
> --- a/fs/ext4/inline.c
> +++ b/fs/ext4/inline.c
> @@ -502,9 +502,8 @@ static int ext4_read_inline_folio(struct inode *inode, struct folio *folio)
>         BUG_ON(len > PAGE_SIZE);
>         kaddr = kmap_local_folio(folio, 0);
>         ret = ext4_read_inline_data(inode, kaddr, len, &iloc);
> -       flush_dcache_folio(folio);
> +       kaddr = folio_zero_tail(folio, len, kaddr + len);
>         kunmap_local(kaddr);
> -       folio_zero_segment(folio, len, folio_size(folio));
>         folio_mark_uptodate(folio);
>         brelse(iloc.bh);
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 4cacc0e43b51..1b81416196dd 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -483,6 +483,44 @@ static inline void memcpy_to_folio(struct folio *folio, size_t offset,
>         flush_dcache_folio(folio);
>  }
>
> +/**
> + * folio_zero_tail - Zero the tail of a folio.
> + * @folio: The folio to zero.
> + * @kaddr: The address the folio is currently mapped to.
> + * @offset: The byte offset in the folio to start zeroing at.
> + *

As Andrew has pointed out, the order of the arguments in the
description doesn't match the order in the function definition. Other
than that, this patch looks good, so

Reviewed-by: Andreas Gruenbacher <agruenba at redhat.com>

> + * If you have already used kmap_local_folio() to map a folio, written
> + * some data to it and now need to zero the end of the folio (and flush
> + * the dcache), you can use this function.  If you do not have the
> + * folio kmapped (eg the folio has been partially populated by DMA),
> + * use folio_zero_range() or folio_zero_segment() instead.
> + *
> + * Return: An address which can be passed to kunmap_local().
> + */
> +static inline __must_check void *folio_zero_tail(struct folio *folio,
> +               size_t offset, void *kaddr)
> +{
> +       size_t len = folio_size(folio) - offset;
> +
> +       if (folio_test_highmem(folio)) {
> +               size_t max = PAGE_SIZE - offset_in_page(offset);
> +
> +               while (len > max) {
> +                       memset(kaddr, 0, max);
> +                       kunmap_local(kaddr);
> +                       len -= max;
> +                       offset += max;
> +                       max = PAGE_SIZE;
> +                       kaddr = kmap_local_folio(folio, offset);
> +               }
> +       }
> +
> +       memset(kaddr, 0, len);
> +       flush_dcache_folio(folio);
> +
> +       return kaddr;
> +}
> +
>  /**
>   * memcpy_from_file_folio - Copy some bytes from a file folio.
>   * @to: The destination buffer.
> --
> 2.42.0
>

Thanks,
Andreas