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

cuiweixie
2026-02-24T12:51:41Z

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>
---
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);
+ }
} 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)