[PATCH v4] zram: Fix the issue that the write - back limits might overflow

[Yuwen Chen <ywen.chen@foxmail.com>]

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.

Fixes: 1d69a3f8ae77e ("zram: idle writeback fixes and cleanup")
Signed-off-by: Yuwen Chen <ywen.chen@foxmail.com>
---
Changes in v4:
  - Fix the code format issue.
Changes in v3:
  - Simplify the code using rounddown.
Changes in v2:
  - Rebase the patch to adapt to the latest version.

 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 8a13729..fff9e45 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -579,6 +579,16 @@ 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);
 	zram->bd_wb_limit = val;
 	up_write(&zram->init_lock);
-- 
2.34.1