lz4_zfs: FreeBSD-local changes to use a custom allocator.

This is not associated with a specific upstream commit but apparently
a local diff applied as part of:

commit e92ffd9b626833ebdbf2742c8ffddc6cd94b963e
Merge: 3c3df3660072 17b2ae0
Author: Martin Matuska <mm@FreeBSD.org>
Date:   Sat Jan 22 23:05:15 2022 +0100

    zfs: merge openzfs/zfs@17b2ae0b2 (master) into main

    Notable upstream pull request merges:
      openzfs#12766 Fix error propagation from lzc_send_redacted
      openzfs#12805 Updated the lz4 decompressor
      openzfs#12851 FreeBSD: Provide correct file generation number
      openzfs#12857 Verify dRAID empty sectors
      openzfs#12874 FreeBSD: Update argument types for VOP_READDIR
      openzfs#12896 Reduce number of arc_prune threads
      openzfs#12934 FreeBSD: Fix zvol_*_open() locking
      openzfs#12947 lz4: Cherrypick fix for CVE-2021-3520
      openzfs#12961 FreeBSD: Fix leaked strings in libspl mnttab
      openzfs#12964 Fix handling of errors from dmu_write_uio_dbuf() on FreeBSD
      openzfs#12981 Introduce a flag to skip comparing the local mac when raw sending
      openzfs#12985 Avoid memory allocations in the ARC eviction thread

    Obtained from:  OpenZFS
    OpenZFS commit: 17b2ae0

module/zfs/lz4_zfs.c

@@ -50,7 +50,8 @@ static int LZ4_compress64kCtx(void *ctx, const char *source, char *dest,
 int LZ4_uncompress_unknownOutputSize(const char *source, char *dest,
     int isize, int maxOutputSize);
 
-static kmem_cache_t *lz4_cache;
+static void *lz4_alloc(int flags);
+static void lz4_free(void *ctx);
 
 size_t
 lz4_compress_zfs(void *s_start, void *d_start, size_t s_len,
@@ -842,8 +843,7 @@ real_LZ4_compress(const char *source, char *dest, int isize, int osize)
 	void *ctx;
 	int result;
 
-	ASSERT(lz4_cache != NULL);
-	ctx = kmem_cache_alloc(lz4_cache, KM_SLEEP);
+	ctx = lz4_alloc(KM_SLEEP);
 
 	/*
 	 * out of kernel memory, gently fall through - this will disable
@@ -859,10 +859,29 @@ real_LZ4_compress(const char *source, char *dest, int isize, int osize)
 	else
 		result = LZ4_compressCtx(ctx, source, dest, isize, osize);
 
-	kmem_cache_free(lz4_cache, ctx);
+	lz4_free(ctx);
 	return (result);
 }
 
+#ifdef __FreeBSD__
+/*
+ * FreeBSD has 4, 8 and 16 KB malloc zones which can be used here.
+ * Should struct refTables get resized this may need to be revisited, hence
+ * compiler-time asserts.
+ */
+_Static_assert(sizeof(struct refTables) <= 16384,
+    "refTables too big for malloc");
+_Static_assert((sizeof(struct refTables) % 4096) == 0,
+    "refTables not a multiple of page size");
+#else
+#define ZFS_LZ4_USE_CACHE
+#endif
+
+#ifdef ZFS_LZ4_USE_CACHE
+static kmem_cache_t *lz4_cache;
+#endif
+
+#ifdef ZFS_LZ4_USE_CACHE
 void
 lz4_init(void)
 {
@@ -878,3 +897,39 @@ lz4_fini(void)
 		lz4_cache = NULL;
 	}
 }
+
+static void *
+lz4_alloc(int flags)
+{
+	ASSERT(lz4_cache != NULL);
+	return (kmem_cache_alloc(lz4_cache, flags));
+}
+
+static void
+lz4_free(void *ctx)
+{
+	kmem_cache_free(lz4_cache, ctx);
+}
+#else
+void
+lz4_init(void)
+{
+}
+
+void
+lz4_fini(void)
+{
+}
+
+static void *
+lz4_alloc(int flags)
+{
+	return (kmem_alloc(sizeof (struct refTables), flags));
+}
+
+static void
+lz4_free(void *ctx)
+{
+	kmem_free(ctx, sizeof (struct refTables));
+}
+#endif