hmm… prolly makes sense to default to the 64K-compatible setting for jemalloc then, just to cover more systems. as said, the 64K setting should work on 4K systems? it might just not be optimal (at least according to an upstream issue on raising the aarch64 default from 4K to 64K).

a function argument to override that would prolly still be nice though - could be interesting for someone measuring the performance impact and deciding to optimise for their system via an overlay.

diff --git a/pkgs/by-name/je/jemalloc/package.nix b/pkgs/by-name/je/jemalloc/package.nix
index 202195c152db..88e7b776b28d 100644
--- a/pkgs/by-name/je/jemalloc/package.nix
+++ b/pkgs/by-name/je/jemalloc/package.nix
@@ -13,6 +13,18 @@
   # compatibility.
   stripPrefix ? stdenv.hostPlatform.isDarwin,
   disableInitExecTls ? false,
+  # The upstream default is dependent on the builders' page size
+  # https://github.com/jemalloc/jemalloc/issues/467
+  # https://sources.debian.org/src/jemalloc/5.3.0-3/debian/rules/
+  pageSizeLog2 ?
+    if
+      (
+        stdenv.hostPlatform.isAarch64 || stdenv.hostPlatform.isLoongArch64 || stdenv.hostPlatform.isPower64
+      )
+    then
+      16
+    else
+      12,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -46,21 +58,11 @@ stdenv.mkDerivation (finalAttrs: {
   configureFlags = [
     "--with-version=${lib.versions.majorMinor finalAttrs.version}.0-0-g${finalAttrs.src.rev}"
     "--with-lg-vaddr=${with stdenv.hostPlatform; toString (if isILP32 then 32 else parsed.cpu.bits)}"
+    (lib.strings.withFeatureAs true "lg-page" (toString pageSizeLog2))
   ]
   # see the comment on stripPrefix
   ++ lib.optional stripPrefix "--with-jemalloc-prefix="
   ++ lib.optional disableInitExecTls "--disable-initial-exec-tls"
-  # The upstream default is dependent on the builders' page size
-  # https://github.com/jemalloc/jemalloc/issues/467
-  # https://sources.debian.org/src/jemalloc/5.3.0-3/debian/rules/
-  ++ [
-    (
-      if (stdenv.hostPlatform.isAarch64 || stdenv.hostPlatform.isLoongArch64) then
-        "--with-lg-page=16"
-      else
-        "--with-lg-page=12"
-    )
-  ]
   # See https://github.com/jemalloc/jemalloc/issues/1997
   # Using a value of 48 should work on both emulated and native x86_64-darwin.
   ++ lib.optional (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isx86_64) "--with-lg-vaddr=48";

ig

if we reject non-int values for pageSizeKiB with an assert + informative message, then supplying an unsupported number actually displays the supported options on my end (Lix)

       error: attribute '"32"' missing
       at /home/puna/Development/nixpkgs/pkgs/by-name/je/jemalloc/package.nix:76:10:
           75|         }
           76|         ."${toString pageSizeKiB}"
             |          ^
           77|     }"

       note: trace involved the following derivations:
       derivation 'jemalloc-5.3.0-unstable-2025-09-12'

       Did you mean one of 16, 4 or 64?

maybe useful, prolly overkill. thoughts?

if we reject non-int values for pageSizeKiB with an assert + informative message at the start, then supplying an unsupported number actually displays the supported options on my end (Lix)

       error: attribute '"32"' missing
       at /home/puna/Development/nixpkgs/pkgs/by-name/je/jemalloc/package.nix:76:10:
           75|         }
           76|         ."${toString pageSizeKiB}"
             |          ^
           77|     }"

       note: trace involved the following derivations:
       derivation 'jemalloc-5.3.0-unstable-2025-09-12'

       Did you mean one of 16, 4 or 64?

maybe useful, prolly overkill. thoughts?

error:
       … while evaluating list element at index 2

       … while calling the 'toString' builtin
         at /home/amaanq/projects/nix/nixpkgs/pkgs/by-name/je/jemalloc/package.nix:70:7:
           69|     "--with-lg-page=${
           70|       toString
             |       ^
           71|         {

       error: attribute '"32"' missing
       at /home/amaanq/projects/nix/nixpkgs/pkgs/by-name/je/jemalloc/package.nix:71:9:
           70|       toString
           71|         {
             |         ^
           72|           "4" = 12;
       Did you mean one of 16, 4 or 64?

oh, way to go is prolly

let
  pageSizeMap = { 
    "4" = 12; 
    "16" = 14; 
    "64" = 16; 
  };  
in
assert lib.asserts.assertOneOf "pageSizeKiB" (toString pageSizeKiB) (
  builtins.attrNames pageSizeMap
);

assert lib.asserts.assertMsg (builtins.isInt pageSizeKiB)
  "jemalloc: pageSizeKiB must be an integer (one of 4, 16, or 64), got: ${toString pageSizeKiB}";

assert lib.asserts.assertMsg (builtins.isInt pageSizeKiB)
  "jemalloc: pageSizeKiB must be an integer (one of 4, 16, or 64), got: ${toString pageSizeKiB}";

       error: pageSizeKiB must be one of [
         "16"
         "4"
         "64"
       ], but is: "bla"