linux-kernel - Re: [PATCH 1/1] mm: bdi: Initialize bdi_min

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Date:   Fri, 10 Dec 2021 22:36:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Manjong Lee <mj0123.lee@...sung.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, seunghwan.hyun@...sung.com,
        sookwan7.kim@...sung.com, nanich.lee@...sung.com,
        yt0928.kim@...sung.com, junho89.kim@...sung.com,
        jisoo2146.oh@...sung.com
Subject: Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

On Thu, Oct 21, 2021 at 07:45:30PM -0700, Andrew Morton wrote:
> On Thu, 21 Oct 2021 13:01:30 +0100 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Thu, Oct 21, 2021 at 04:16:52AM -0700, Christoph Hellwig wrote:
> > > On Fri, Oct 22, 2021 at 01:19:43AM +0900, Manjong Lee wrote:
> > > > Because when sdcard is removed, bdi_min_ratio value will remain.
> > > > Currently, the only way to reset bdi_ min_ratio is to reboot.
> > > 
> > > But bdis that are unregistered are never re-registered.  What is
> > > the problem you're trying to solve?
> > 
> > The global bdi_min_ratio needs to be adjusted.  See
> > bdi_set_min_ratio() in mm/page-writeback.c.
> 
> I added cc:stable to this and tweaked the comment & coding style a bit:

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> --- a/mm/backing-dev.c~mm-bdi-initialize-bdi_min_ratio-when-bdi-unregister-fix
> +++ a/mm/backing-dev.c
> @@ -947,10 +947,12 @@ void bdi_unregister(struct backing_dev_i
>  	wb_shutdown(&bdi->wb);
>  	cgwb_bdi_unregister(bdi);
>  
> -	/* if min ratio doesn't 0, it has to set 0 before unregister */
> -	if (bdi->min_ratio) {
> +	/*
> +	 * If this BDI's min ratio has been set, use bdi_set_min_ratio() to
> +	 * update the global bdi_min_ratio.
> +	 */
> +	if (bdi->min_ratio)
>  		bdi_set_min_ratio(bdi, 0);
> -	}
>  
>  	if (bdi->dev) {
>  		bdi_debug_unregister(bdi);
> _
>