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

[Chi Zhiling <chizhiling@163.com>]

On 2/5/26 09:39, Namjae Jeon wrote:
> On Wed, Feb 4, 2026 at 4:15 PM Chi Zhiling <chizhiling@163.com> wrote:
>>
>> From: Chi Zhiling <chizhiling@kylinos.cn>
>>
>> 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@kylinos.cn>
>> ---
>>   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?

OK, I will do this in v2.

> Thanks.