[PATCH v2] zram: do not slot_free() written-back slots

[Sergey Senozhatsky <senozhatsky@chromium.org>]

slot_free() basically completely resets the slots by clearing
all of its flags and attributes.  While zram_writeback_complete()
restores some of flags back (those that are necessary for async
read decompression) we still lose a lot of slot's metadata.  For
example, slot's ac-time, or ZRAM_INCOMPRESSIBLE.

More importantly, restoring flags/attrs requires extra attention
as some of the flags are directly affecting zram device stats.
And the original code did not pay that attention.  Namely ZRAM_HUGE
slots handling in zram_writeback_complete().  The call to slot_free()
would decrement ->huge_pages, however when zram_writeback_complete()
restored the slot's ZRAM_HUGE flag, it would not get reflected in
an incremented ->huge_pages.  So when the slot would finally get freed,
slot_free() would decrement ->huge_pages again, leading to underflow.

Fix this by open-coding the required memory free and stats updates
in zram_writeback_complete(), rather than calling the destructive
slot_free().  Since we now preserve the ZRAM_HUGE flag on written-back
slots (for the deferred decompression path), we also update
slot_free() to skip decrementing ->huge_pages if ZRAM_WB is set.

Fixes: d38fab605c667 ("zram: introduce compressed data writeback")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---

v1 - >v2:
- Updated commit message.

 drivers/block/zram/zram_drv.c | 39 +++++++++++++----------------------
 1 file changed, 14 insertions(+), 25 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 1b0e1c02ece7..5ecc4ba40e9d 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -917,9 +917,8 @@ static void zram_account_writeback_submit(struct zram *zram)
 
 static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 {
-	u32 size, index = req->pps->index;
-	int err, prio;
-	bool huge;
+	u32 index = req->pps->index;
+	int err;
 
 	err = blk_status_to_errno(req->bio.bi_status);
 	if (err) {
@@ -946,28 +945,13 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 		goto out;
 	}
 
-	if (zram->compressed_wb) {
-		/*
-		 * ZRAM_WB slots get freed, we need to preserve data required
-		 * for read decompression.
-		 */
-		size = get_slot_size(zram, index);
-		prio = get_slot_comp_priority(zram, index);
-		huge = test_slot_flag(zram, index, ZRAM_HUGE);
-	}
-
-	slot_free(zram, index);
-	set_slot_flag(zram, index, ZRAM_WB);
+	clear_slot_flag(zram, index, ZRAM_IDLE);
+	if (test_slot_flag(zram, index, ZRAM_HUGE))
+		atomic64_dec(&zram->stats.huge_pages);
+	atomic64_sub(get_slot_size(zram, index), &zram->stats.compr_data_size);
+	zs_free(zram->mem_pool, get_slot_handle(zram, index));
 	set_slot_handle(zram, index, req->blk_idx);
-
-	if (zram->compressed_wb) {
-		if (huge)
-			set_slot_flag(zram, index, ZRAM_HUGE);
-		set_slot_size(zram, index, size);
-		set_slot_comp_priority(zram, index, prio);
-	}
-
-	atomic64_inc(&zram->stats.pages_stored);
+	set_slot_flag(zram, index, ZRAM_WB);
 
 out:
 	slot_unlock(zram, index);
@@ -2030,8 +2014,13 @@ static void slot_free(struct zram *zram, u32 index)
 	set_slot_comp_priority(zram, index, 0);
 
 	if (test_slot_flag(zram, index, ZRAM_HUGE)) {
+		/*
+		 * Writeback completion decrements ->huge_pages but keeps
+		 * ZRAM_HUGE flag for deferred decompression path.
+		 */
+		if (!test_slot_flag(zram, index, ZRAM_WB))
+			atomic64_dec(&zram->stats.huge_pages);
 		clear_slot_flag(zram, index, ZRAM_HUGE);
-		atomic64_dec(&zram->stats.huge_pages);
 	}
 
 	if (test_slot_flag(zram, index, ZRAM_WB)) {
-- 
2.53.0.959.g497ff81fa9-goog