[PATCH v4 2/3] mm/zswap: remove the memcpy if acomp is not sleepable

[Barry Song <21cnbao@gmail.com>]

From: Barry Song <v-songbaohua@oppo.com>

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 the algorithm flags CRYPTO_ALG_ASYNC to know
if acomp is sleepable.
Generally speaking, async and sleepable are semantically similar
but not equal. But for compress drivers, they are actually 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.

Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 mm/zswap.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 51de79aa8659..115a780fa2c1 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -162,6 +162,7 @@ struct crypto_acomp_ctx {
 	struct crypto_wait wait;
 	u8 *buffer;
 	struct mutex mutex;
+	bool is_sleepable;
 };
 
 /*
@@ -973,6 +974,8 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
 		goto acomp_fail;
 	}
 	acomp_ctx->acomp = acomp;
+	acomp_ctx->is_sleepable = crypto_acomp_get_alg_flags(acomp) &
+				  CRYPTO_ALG_ASYNC;
 
 	req = acomp_request_alloc(acomp_ctx->acomp);
 	if (!req) {
@@ -1100,7 +1103,7 @@ 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);
@@ -1114,7 +1117,7 @@ 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);
 }
 
-- 
2.34.1