linux-kernel - Re: [PATCH v2] mm: fix ratelimit_pages update error in dirty_ratio

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Message-Id: <20250415185851.e8d632f60ec5049f734ac2a8@linux-foundation.org>
Date: Tue, 15 Apr 2025 18:58:51 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: alexjlzheng@...il.com
Cc: willy@...radead.org, andrea@...terlinux.com, fengguang.wu@...el.com,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, mengensun@...cent.com, Jinliang Zheng
 <alexjlzheng@...cent.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm: fix ratelimit_pages update error in
 dirty_ratio_handler()

On Tue, 15 Apr 2025 17:02:32 +0800 alexjlzheng@...il.com wrote:

> From: Jinliang Zheng <alexjlzheng@...cent.com>
> 
> In the dirty_ratio_handler() function, 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.

Can you please tell us precisely where global_dirty_limits()
prioritizes vm_dirty_bytes?  I spent a while chasing code and didn't
see how global_dirty_limits() gets to node_dirty_ok()(?).

> That causes ratelimit_pages to still use the value calculated based on
> vm_dirty_bytes, which is wrong now.
> 
> Fixes: 9d823e8f6b1b ("writeback: per task dirty rate limit")
> Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> Reviewed-by: MengEn Sun <mengensun@...cent.com>
> Cc: stable@...r.kernel.org

Please, as always, provide a description of the userspace-visible
effects of this bug?