+ zram-fix-the-issue-that-the-write-back-limits-might-overflow.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: zram: fix the issue that the write - back limits might overflow
has been added to the -mm mm-new branch.  Its filename is
     zram-fix-the-issue-that-the-write-back-limits-might-overflow.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zram-fix-the-issue-that-the-write-back-limits-might-overflow.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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patches, and acceptance into mm-new is a notification for others take
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------------------------------------------------------
From: Yuwen Chen <ywen.chen@foxmail.com>
Subject: zram: fix the issue that the write - back limits might overflow
Date: Wed, 19 Nov 2025 12:13:45 +0800

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: 1d69a3f8ae77e ("zram: idle writeback fixes and cleanup")
Signed-off-by: Yuwen Chen <ywen.chen@foxmail.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Richard Chang <richardycc@google.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/block/zram/zram_drv.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/drivers/block/zram/zram_drv.c~zram-fix-the-issue-that-the-write-back-limits-might-overflow
+++ a/drivers/block/zram/zram_drv.c
@@ -564,6 +564,16 @@ static ssize_t writeback_limit_store(str
 	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);
_

Patches currently in -mm which might be from ywen.chen@foxmail.com are

zram-introduce-writeback-bio-batching-support.patch
zram-fix-the-issue-that-the-write-back-limits-might-overflow.patch