linux-kernel - [PATCH] zram: Fix __zram_bvec

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Date:   Mon, 19 Oct 2020 12:13:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org,
        Mike Galbraith <umgwanakikbuti@...il.com>, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, bigeasy@...utronix.de,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] zram: Fix __zram_bvec_{read,write}() locking order


Mikhail reported a lockdep spat detailing how __zram_bvec_read() and
__zram_bvec_write() use zstrm->lock and zspage->lock in opposite order.

Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>
---
 drivers/block/zram/zram_drv.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 9100ac36670a..c1e2c2e1cde8 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1216,10 +1216,11 @@ static void zram_free_page(struct zram *zram, size_t index)
 static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
 				struct bio *bio, bool partial_io)
 {
-	int ret;
+	struct zcomp_strm *zstrm;
 	unsigned long handle;
 	unsigned int size;
 	void *src, *dst;
+	int ret;
 
 	zram_slot_lock(zram, index);
 	if (zram_test_flag(zram, index, ZRAM_WB)) {
@@ -1250,6 +1251,9 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
 
 	size = zram_get_obj_size(zram, index);
 
+	if (size != PAGE_SIZE)
+		zstrm = zcomp_stream_get(zram->comp);
+
 	src = zs_map_object(zram->mem_pool, handle, ZS_MM_RO);
 	if (size == PAGE_SIZE) {
 		dst = kmap_atomic(page);
@@ -1257,8 +1261,6 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
 		kunmap_atomic(dst);
 		ret = 0;
 	} else {
-		struct zcomp_strm *zstrm = zcomp_stream_get(zram->comp);
-
 		dst = kmap_atomic(page);
 		ret = zcomp_decompress(zstrm, src, size, dst);
 		kunmap_atomic(dst);