Function zstd_dctx_alloc

freebsd/contrib/openzfs/module/zstd/zfs_zstd.c:574–616

* Allocator for zstd decompression context using mempool_allocator with * fallback to reserved memory if allocation fails */

 * fallback to reserved memory if allocation fails
 */
static void *
zstd_dctx_alloc(void *opaque __maybe_unused, size_t size)
{
	size_t nbytes = sizeof (struct zstd_kmem) + size;
	struct zstd_kmem *z = NULL;
	enum zstd_kmem_type type = ZSTD_KMEM_DEFAULT;

	z = (struct zstd_kmem *)zstd_mempool_alloc(zstd_mempool_dctx, nbytes);
	if (!z) {
		/* Try harder, decompression shall not fail */
		z = vmem_alloc(nbytes, KM_SLEEP);
		if (z) {
			z->pool = NULL;
		}
		ZSTDSTAT_BUMP(zstd_stat_alloc_fail);
	} else {
		return ((void*)z + (sizeof (struct zstd_kmem)));
	}

	/* Fallback if everything fails */
	if (!z) {
		/*
		 * Barrier since we only can handle it in a single thread. All
		 * other following threads need to wait here until decompression
		 * is completed. zstd_free will release this barrier later.
		 */
		mutex_enter(&zstd_dctx_fallback.barrier);

		z = zstd_dctx_fallback.mem;
		type = ZSTD_KMEM_DCTX;
		ZSTDSTAT_BUMP(zstd_stat_alloc_fallback);
	}

	/* Allocation should always be successful */
	if (!z) {
		return (NULL);
	}

	z->kmem_type = type;
	z->kmem_size = nbytes;

	return ((void*)z + (sizeof (struct zstd_kmem)));
}

/* Free allocated memory by its specific type */
static void