mm/zswap: remove the memcpy if acomp is not sleepable

Most compressors are actually CPU-based and won't sleep during compression
and decompression.  We should remove the redundant memcpy for them.

This patch checks if the algorithm is sleepable by testing the
CRYPTO_ALG_ASYNC algorithm flag.

Generally speaking, async and sleepable are semantically similar but not
equal.  But for compress drivers, they are basically equal at least due to
the below facts.

Firstly, scompress drivers - crypto/deflate.c, lz4.c, zstd.c, lzo.c etc
have no sleep.  Secondly, zRAM has been using these scompress drivers for
years in atomic contexts, and never worried those drivers going to sleep.

One exception is that an async driver can sometimes still return
synchronously per Herbert's clarification.  In this case, we are still
having a redundant memcpy.  But we can't know if one particular acomp
request will sleep or not unless crypto can expose more details for each
specific request from offload drivers.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Barry Song <[email protected]>
Tested-by: Chengming Zhou <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Acked-by: Chris Li <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/zswap.c

@@ -162,6 +162,7 @@ struct crypto_acomp_ctx {
 	struct crypto_wait wait;
 	u8 *buffer;
 	struct mutex mutex;
+	bool is_sleepable;
 };
 
 /*
@@ -951,6 +952,7 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
 		goto acomp_fail;
 	}
 	acomp_ctx->acomp = acomp;
+	acomp_ctx->is_sleepable = acomp_is_async(acomp);
 
 	req = acomp_request_alloc(acomp_ctx->acomp);
 	if (!req) {
@@ -1078,7 +1080,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
 	mutex_lock(&acomp_ctx->mutex);
 
 	src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
-	if (!zpool_can_sleep_mapped(zpool)) {
+	if (acomp_ctx->is_sleepable && !zpool_can_sleep_mapped(zpool)) {
 		memcpy(acomp_ctx->buffer, src, entry->length);
 		src = acomp_ctx->buffer;
 		zpool_unmap_handle(zpool, entry->handle);
@@ -1092,7 +1094,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
 	BUG_ON(acomp_ctx->req->dlen != PAGE_SIZE);
 	mutex_unlock(&acomp_ctx->mutex);
 
-	if (zpool_can_sleep_mapped(zpool))
+	if (!acomp_ctx->is_sleepable || zpool_can_sleep_mapped(zpool))
 		zpool_unmap_handle(zpool, entry->handle);
 }