6.6-stable review patch. If anyone has any objections, please let me know.
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From: Jinliang Zheng alexjlzheng@tencent.com
commit f83f362d40ccceb647f7d80eb92206733d76a36b upstream.
In dirty_ratio_handler(), vm_dirty_bytes must be set to zero before calling writeback_set_ratelimit(), as global_dirty_limits() always prioritizes the value of vm_dirty_bytes.
It's domain_dirty_limits() that's relevant here, not node_dirty_ok:
dirty_ratio_handler writeback_set_ratelimit global_dirty_limits(&dirty_thresh) <- ratelimit_pages based on dirty_thresh domain_dirty_limits if (bytes) <- bytes = vm_dirty_bytes <--------+ thresh = f1(bytes) <- prioritizes vm_dirty_bytes | else | thresh = f2(ratio) | ratelimit_pages = f3(dirty_thresh) | vm_dirty_bytes = 0 <- it's late! ---------------------+
This causes ratelimit_pages to still use the value calculated based on vm_dirty_bytes, which is wrong now.
The impact visible to userspace is difficult to capture directly because there is no procfs/sysfs interface exported to user space. However, it will have a real impact on the balance of dirty pages.
For example:
1. On default, we have vm_dirty_ratio=40, vm_dirty_bytes=0
2. echo 8192 > dirty_bytes, then vm_dirty_bytes=8192, vm_dirty_ratio=0, and ratelimit_pages is calculated based on vm_dirty_bytes now.
3. echo 20 > dirty_ratio, then since vm_dirty_bytes is not reset to zero when writeback_set_ratelimit() -> global_dirty_limits() -> domain_dirty_limits() is called, reallimit_pages is still calculated based on vm_dirty_bytes instead of vm_dirty_ratio. This does not conform to the actual intent of the user.
Link: https://lkml.kernel.org/r/20250415090232.7544-1-alexjlzheng@tencent.com Fixes: 9d823e8f6b1b ("writeback: per task dirty rate limit") Signed-off-by: Jinliang Zheng alexjlzheng@tencent.com Reviewed-by: MengEn Sun mengensun@tencent.com Cc: Andrea Righi andrea@betterlinux.com Cc: Fenggaung Wu fengguang.wu@intel.com Cc: Jinliang Zheng alexjlzheng@tencent.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/page-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -541,8 +541,8 @@ static int dirty_ratio_handler(struct ct
ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); if (ret == 0 && write && vm_dirty_ratio != old_ratio) { - writeback_set_ratelimit(); vm_dirty_bytes = 0; + writeback_set_ratelimit(); } return ret; }