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

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Date:   Thu, 21 Oct 2021 19:45:30 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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, 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:

--- 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);
_