Re: [PATCH v3] ext4: simplify mballoc preallocation size rounding for small files

Andreas Dilger
2026-02-25T00:01:09Z

On Feb 24, 2026, at 05:51, cuiweixie@gmail.com wrote:
>
> From: Weixie Cui <cuiweixie@gmail.com>
>
> The if-else ladder in ext4_mb_normalize_request() manually rounds up
> the preallocation size to the next power of two for files up to 1MB,
> enumerating each step from 16KB to 1MB individually. Replace this with
> a single roundup_pow_of_two() call clamped to a 16KB minimum, which
> is functionally equivalent but much more concise.
>
> Also replace raw byte constants with SZ_1M and SZ_16K from
> <linux/sizes.h> for clarity, and remove the stale "XXX: should this
> table be tunable?" comment that has been there since the original
> mballoc code.
>
> No functional change.
>
> Signed-off-by: Weixie Cui <cuiweixie@gmail.com>

One minor code style nit below, but otherwise,

Reviewed-by: Andreas Dilger <adilger@dilger.ca <mailto:adilger@dilger.ca>>

> ---
> fs/ext4/mballoc.c | 25 ++++++++++---------------
> 1 file changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 20e9fdaf4301..a5c51daaba78 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4561,22 +4561,17 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
> (req <= (size) || max <= (chunk_size))
>
> /* first, try to predict filesize */
> - /* XXX: should this table be tunable? */
> start_off = 0;
> - if (size <= 16 * 1024) {
> - size = 16 * 1024;
> - } else if (size <= 32 * 1024) {
> - size = 32 * 1024;
> - } else if (size <= 64 * 1024) {
> - size = 64 * 1024;
> - } else if (size <= 128 * 1024) {
> - size = 128 * 1024;
> - } else if (size <= 256 * 1024) {
> - size = 256 * 1024;
> - } else if (size <= 512 * 1024) {
> - size = 512 * 1024;
> - } else if (size <= 1024 * 1024) {
> - size = 1024 * 1024;
> + if (size <= SZ_1M) {
> + /*
> + * For files up to 1MB, round up the preallocation size to
> + * the next power of two, with a minimum of 16KB.
> + */
> + if (size <= (unsigned long)SZ_16K) {
> + size = SZ_16K;
> + } else {
> + size = roundup_pow_of_two(size);
> + }

No need for {} around single-line if-else blocks.

> } else if (NRL_CHECK_SIZE(size, 4 * 1024 * 1024, max, 2 * 1024)) {
> start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
> (21 - bsbits)) << 21;
> --
> 2.39.5 (Apple Git-154)
>