[PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Manjong Lee]

Initialize min_ratio if it is set when bdi unregister.
It can prevent problems that may occur when bdi is removed
without resetting min_ratio.

For example.
1) insert external sdcard
2) set external sdcard's min_ratio 70
3) remove external sdcard without setting min_ratio 0
4) insert external sdcard
5) set external sdcard's min_ratio 70 << error occur(can't set)

Because when sdcard is removed, bdi_min_ratio value will remain.
Currently, the only way to reset bdi_ min_ratio is to reboot.

Signed-off-by: Manjong Lee <mj0123.lee@samsung.com>
---
 mm/backing-dev.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 4a9d4e27d0d9..ca53807be188 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -947,6 +947,11 @@ void bdi_unregister(struct backing_dev_info *bdi)
 	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) {
+		bdi_set_min_ratio(bdi, 0);
+	}
+
 	if (bdi->dev) {
 		bdi_debug_unregister(bdi);
 		device_unregister(bdi->dev);
-- 
2.32.0

Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Christoph Hellwig]

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?

Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Matthew Wilcox]

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.

Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Andrew Morton]

On Thu, 21 Oct 2021 13:01:30 +0100 Matthew Wilcox <willy@infradead.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);
_

Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Matthew Wilcox]

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@infradead.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:

Definitely improvements on that front.

I don't know the BDI code particularly well, and the implementation of
bdi_set_min_ratio() confuses me, so I can't say whether the original
patch is clearly correct or not.

Re:Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Manjong Lee]

>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@infradead.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:
>
>Definitely improvements on that front.
>
>I don't know the BDI code particularly well, and the implementation of
>bdi_set_min_ratio() confuses me, so I can't say whether the original
>patch is clearly correct or not.


Thank Andrew Morton for modifying the comment & style. :)

Dear Matthew Wilcox. 
Please tell me about it more? I want to know more, 
and I want to find a way to improve it. :)

Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Peter Zijlstra]

On Fri, Oct 22, 2021 at 03:58:13AM +0100, Matthew Wilcox wrote:
> 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@infradead.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:
> 
> Definitely improvements on that front.
> 
> I don't know the BDI code particularly well, and the implementation of
> bdi_set_min_ratio() confuses me, so I can't say whether the original
> patch is clearly correct or not.

Looks like something I might've written :-)

I'm thinking the part:

	min_ratio -= bdi->min_ratio;
	if (bdi_min_ratio + min_ratio < 100) {
		bdi_min_ratio += min_ratio;
		bdi->min_ratio += min_ratio;
	}

is what confuses? particularly when min_ratio < bdi->min_ratio?

Anyway, two cases: bdi->min_ratio < min_ratio and bdi->min_ratio >
min_ratio, let's do both with 0 and 10.

bdi->min_ratio = 0, min_ratio = 10 gives us:

min_ratio -= bdi->min_ratio;		// 10 -= 0 == 10
if (bdi_min_ratio + min_ratio < 100) {	// x + 10 < 100
	bdi_min_ratio += min_ratio;	// x += 10
	bdi->min_ratio += min_ratio	// 0 += 10 == 10
}

The other way around, bdi->min_raito = 10, min_ratio = 0:

min_ratio -= bdi->min_ratio;		// 0 -= 10 == -10
if (bdi_min_ratio + min_ratio < 100) {	// x + -10 < 100
	bdi_min_ratio += min_ratio;	// x += -10
	bdi->min_ratio += min_ratio;	// 10 += -10 == 0
}

Makes sense?

Re: [PATCH 1/1] mm: bdi: Initialize bdi_min_ratio when bdi unregister

[Peter Zijlstra]

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@infradead.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@infradead.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);
> _
>