Re: [PATCH v1 1/3] exfat: add block readahead in exfat_chain_cont_cluster

[Namjae Jeon]

On Wed, Feb 4, 2026 at 4:15 PM Chi Zhiling <chizhiling@xxxxxxx> wrote:
>
> From: Chi Zhiling <chizhiling@xxxxxxxxxx>
>
> The conversion from NO_FAT_CHAIN format to FAT_CHAIN format occurs
> when the file cannot allocate contiguous space. When the file to be
> converted is very large, this process can take a long time.
>
> This patch introduces simple readahead to read all the blocks in
> advance, as these blocks are consecutive.
>
> Test in an empty exfat filesystem:
> dd if=/dev/zero of=/mnt/file bs=1M count=30k
> dd if=/dev/zero of=/mnt/file2 bs=1M count=1
> time cat /mnt/file2 >> /mnt/file
>
> | cluster size | before patch | after patch |
> | ------------ | ------------ | ----------- |
> | 512          | 47.667s      | 4.316s      |
> | 4k           | 6.436s       | 0.541s      |
> | 32k          | 0.758s       | 0.071s      |
> | 256k         | 0.117s       | 0.011s      |
>
> Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
> ---
>  fs/exfat/exfat_fs.h |  9 +++++++--
>  fs/exfat/fatent.c   | 38 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+), 2 deletions(-)
>
> diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
> index 2dbed5f8ec26..5a3cdf725846 100644
> --- a/fs/exfat/exfat_fs.h
> +++ b/fs/exfat/exfat_fs.h
> @@ -10,6 +10,7 @@
>  #include <linux/ratelimit.h>
>  #include <linux/nls.h>
>  #include <linux/blkdev.h>
> +#include <linux/backing-dev.h>
>  #include <uapi/linux/exfat.h>
>
>  #define EXFAT_ROOT_INO         1
> @@ -79,6 +80,10 @@ enum {
>  #define EXFAT_HINT_NONE                -1
>  #define EXFAT_MIN_SUBDIR       2
>
> +#define EXFAT_BLK_RA_SIZE(sb)          \
> +(min((sb)->s_bdi->ra_pages, (sb)->s_bdi->io_pages) \
> +        << (PAGE_SHIFT - sb->s_blocksize_bits))
> +
>  /*
>   * helpers for cluster size to byte conversion.
>   */
> @@ -117,9 +122,9 @@ enum {
>  #define FAT_ENT_SIZE (4)
>  #define FAT_ENT_SIZE_BITS (2)
>  #define FAT_ENT_OFFSET_SECTOR(sb, loc) (EXFAT_SB(sb)->FAT1_start_sector + \
> -       (((u64)loc << FAT_ENT_SIZE_BITS) >> sb->s_blocksize_bits))
> +       (((u64)(loc) << FAT_ENT_SIZE_BITS) >> sb->s_blocksize_bits))
>  #define FAT_ENT_OFFSET_BYTE_IN_SECTOR(sb, loc) \
> -       ((loc << FAT_ENT_SIZE_BITS) & (sb->s_blocksize - 1))
> +       (((loc) << FAT_ENT_SIZE_BITS) & (sb->s_blocksize - 1))
>
>  /*
>   * helpers for bitmap.
> diff --git a/fs/exfat/fatent.c b/fs/exfat/fatent.c
> index 71ee16479c43..0c17621587d5 100644
> --- a/fs/exfat/fatent.c
> +++ b/fs/exfat/fatent.c
> @@ -142,13 +142,51 @@ int exfat_ent_get(struct super_block *sb, unsigned int loc,
>         return -EIO;
>  }
>
> +static int exfat_blk_readahead(struct super_block *sb, sector_t sec,
> +               sector_t *ra, blkcnt_t *ra_cnt, sector_t end)
> +{
> +       struct blk_plug plug;
> +
> +       if (sec < *ra)
> +               return 0;
> +
> +       *ra += *ra_cnt;
> +
> +       /* No blocks left (or only the last block), skip readahead. */
> +       if (*ra >= end)
> +               return 0;
> +
> +       *ra_cnt = min(end - *ra + 1, EXFAT_BLK_RA_SIZE(sb));
> +       if (*ra_cnt == 0) {
> +               /* Move 'ra' to the end to disable readahead. */
> +               *ra = end;
> +               return 0;
> +       }
> +
> +       blk_start_plug(&plug);
> +       for (unsigned int i = 0; i < *ra_cnt; i++)
> +               sb_breadahead(sb, *ra + i);
> +       blk_finish_plug(&plug);
> +       return 0;
> +}
Can you combine multiple readahead codes (readahead in
exfat_allocate_bitmap, exfat_dir_readahead) in exfat?
Thanks.