Re: [PATCH v2] exfat: speed up file creation in large directories

[Chi Zhiling <chizhiling@163.com>]

On 8/31/26 11:15 PM, Yang Wen wrote:
> Negative name lookups scan a directory from the beginning. Repeating
> this scan before creating each file makes bulk file creation approach
> O(N^2).
> 
> Add a 64 KiB per-directory Bloom filter and retain the next empty entry
> found during directory scans. A definite Bloom filter miss skips the
> directory scan, while a possible match falls back to the normal lookup
> path, preserving correctness in the presence of hash collisions.
> 
> To avoid overhead for small directories, build the filter lazily when a
> directory reaches 1024 on-disk directory entries. A filter reclaimed
> under memory pressure is rebuilt on demand.
> 
> Manage the filters using a per-superblock LRU and shrinker so that filters
> for inactive directories can be reclaimed under memory pressure.
> 
> Test environment:
>    QEMU TCG multi-thread, 4 vCPUs, 6 GiB RAM
>    4 GiB exFAT image, 32 KiB clusters
> 
> Test script:
>    mount -t exfat /dev/vda /mnt/test
>    time sh -c '
>            i=0
>            while [ "$i" -lt 20000 ]; do
>                    : > "/mnt/test/f.$i" || exit 1
>                    i=$((i + 1))
>            done
>    '
> 
> The measured results were:
> 
>                           Before          After
>    real                   589.48 s         5.33 s
>    user                     4.72 s         2.28 s
>    sys                    584.63 s         3.03 s
> 
> The elapsed time was reduced by 99.10%, from 589.48 seconds to
> 5.33 seconds, corresponding to a 110.6-fold speedup.
> 
> Signed-off-by: Yang Wen <anmuxixixi@gmail.com>
> ---
> 
> Changes in v2:
> - Move exfat_name_filter_free() from exfat_free_inode() to
>    exfat_evict_inode(), after truncate_inode_pages_final(), because
>    ->free_inode() may run from an RCU callback in softirq context.
> - Link to v1: https://lore.kernel.org/all/20260828145838.1187356-1-anmuxixixi@gmail.com/
> ---
>   fs/exfat/dir.c      | 280 ++++++++++++++++++++++++++++++++++++++++++++
>   fs/exfat/exfat_fs.h |  22 ++++
>   fs/exfat/inode.c    |   1 +
>   fs/exfat/namei.c    |  44 ++++++-
>   fs/exfat/super.c    |   9 ++
>   5 files changed, 355 insertions(+), 1 deletion(-)

This is a fairly large patch, which makes the review a bit more
challenging. Perhaps next time you could consider splitting it into
multiple patches in a patch series :)

Reviewed-by: Chi Zhiling <chizhiling@kylinos.cn>