[PATCH 6.6.y 2/4] zram: fix the issue that the write - back limits might overflow

[Sasha Levin <sashal@kernel.org>]

From: Yuwen Chen <ywen.chen@foxmail.com>

[ Upstream commit 04d31610a7221cca624646241b1f6b3edd6c99fd ]

When the page size exceeds 4KB, if bd_wb_limit is set to a value that is
not aligned with the page size, it will cause a numerical wrap-around
issue for bd_wb_limit.  For example, when the page size is set to 16KB and
bd_wb_limit is set to 3, after one write-back operation, the value of
bd_wb_limit will become -1.  More seriously, since bd_wb_limit is an
unsigned number, its value may become as large as 2^64 - 1.

The core reason for this problem is that the unit of bd_wb_limit is 4KB.
For example, when a write-back occurs on a system with a page size of
16KB, 4 needs to be subtracted from bd_wb_limit.  This operation takes
place in the zram_account_writeback_submit function.

This patch fixes the issue by limiting bd_wb_limit to be an integer
multiple of PAGE_SIZE / 4096.

Link: https://lkml.kernel.org/r/tencent_5936CFE72BAB2BA76887BB69DCC1B5E67C05@qq.com
Fixes: 1d69a3f8ae77 ("zram: idle writeback fixes and cleanup")
Signed-off-by: Yuwen Chen <ywen.chen@foxmail.com>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: dde75313eed0 ("zram: set default primary compressor in zram_destroy_comps()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/block/zram/zram_drv.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 8e99f453371e3..64a1136447cf5 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -412,6 +412,16 @@ static ssize_t writeback_limit_store(struct device *dev,
 	if (kstrtoull(buf, 10, &val))
 		return ret;
 
+	/*
+	 * When the page size is greater than 4KB, if bd_wb_limit is set to
+	 * a value that is not page - size aligned, it will cause value
+	 * wrapping. For example, when the page size is set to 16KB and
+	 * bd_wb_limit is set to 3, a single write - back operation will
+	 * cause bd_wb_limit to become -1. Even more terrifying is that
+	 * bd_wb_limit is an unsigned number.
+	 */
+	val = rounddown(val, PAGE_SIZE / 4096);
+
 	down_write(&zram->init_lock);
 	spin_lock(&zram->wb_limit_lock);
 	zram->bd_wb_limit = val;
-- 
2.53.0