mm: fix ratelimit_pages update error in dirty_ratio_handler()

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: 9d823e8f ("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>