Re: [PATCH v2] mm: fix ratelimit_pages update error in dirty_ratio_handler()

[Andrew Morton]

On Tue, 15 Apr 2025 17:02:32 +0800 alexjlzheng@xxxxxxxxx wrote:

> From: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx>
> 
> 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@xxxxxxxxxxx>
> Reviewed-by: MengEn Sun <mengensun@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

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