Reapply "muffin: test, move to 6.18"

This reverts commit 86cf624027.

modules/zfs.nix

@@ -20,7 +20,14 @@ let
   arcMaxBytes = (totalRamBytes - totalHugepageBytes) * 60 / 100;
 in
 {
-  boot.zfs.package = pkgs.zfs_2_4;
+  # remove inline dbuf_evict_one call so the dedicated eviction thread
+  # handles cache pressure instead of stalling txg_sync on 6.14+.
+  # https://github.com/openzfs/zfs/issues/18426
+  boot.zfs.package = pkgs.zfs_2_4.overrideAttrs (old: {
+    patches = (old.patches or [ ]) ++ [
+      ../patches/zfs/0001-remove-dbuf_evict_one-call.patch
+    ];
+  });
   boot.initrd.kernelModules = [ "zfs" ];
 
   boot.kernelParams = [

patches/zfs/0001-remove-dbuf_evict_one-call.patch

@@ -0,0 +1,34 @@
+--- a/module/zfs/dbuf.c	2026-04-24 14:20:12.565390796 -0400
++++ b/module/zfs/dbuf.c	2026-04-24 14:20:40.711198653 -0400
+@@ -864,23 +864,16 @@
+ dbuf_evict_notify(uint64_t size)
+ {
+ 	/*
+-	 * We check if we should evict without holding the dbuf_evict_lock,
+-	 * because it's OK to occasionally make the wrong decision here,
+-	 * and grabbing the lock results in massive lock contention.
++	 * Wake the dedicated eviction thread when the cache
++	 * exceeds target. Inline eviction from write-completion
++	 * context is intentionally avoided: it adds a second
++	 * multilist sublist lock to every write, and when
++	 * kmem_cache_free is slow (__free_frozen_pages zone lock
++	 * contention on 6.14+), the cascade stalls txg_sync.
++	 * See openzfs#18426.
+ 	 */
+-	if (size > dbuf_cache_target_bytes()) {
+-		/*
+-		 * Avoid calling dbuf_evict_one() from memory reclaim context
+-		 * (e.g. Linux kswapd, FreeBSD pagedaemon) to prevent deadlocks.
+-		 * Memory reclaim threads can get stuck waiting for the dbuf
+-		 * hash lock.
+-		 */
+-		if (size > dbuf_cache_hiwater_bytes() &&
+-		    !current_is_reclaim_thread()) {
+-			dbuf_evict_one();
+-		}
++	if (size > dbuf_cache_target_bytes())
+ 		cv_signal(&dbuf_evict_cv);
+-	}
+ }
+ 
+ /*