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

[Andreas Dilger]

On Feb 24, 2026, at 05:51, cuiweixie@xxxxxxxxx wrote:
> 
> From: Weixie Cui <cuiweixie@xxxxxxxxx>
> 
> 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@xxxxxxxxx>

One minor code style nit below, but otherwise,

Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx <mailto:adilger@xxxxxxxxx>>

> ---
> 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)
>