linux-kernel - Re: [PATCH] mm: add strictlimit knob

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Message-Id: <20131104140104.7936d263258a7a6753eb325e@linux-foundation.org>
Date:	Mon, 4 Nov 2013 14:01:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Maxim Patlasov <MPatlasov@...allels.com>
Cc:	karl.kiniger@....ge.com, jack@...e.cz,
	linux-kernel@...r.kernel.org, t.artem@...os.com,
	linux-mm@...ck.org, mgorman@...e.de, tytso@....edu,
	fengguang.wu@...el.com, torvalds@...ux-foundation.org,
	mpatlasov@...allels.com
Subject: Re: [PATCH] mm: add strictlimit knob

On Fri, 01 Nov 2013 18:31:40 +0400 Maxim Patlasov <MPatlasov@...allels.com> wrote:

> "strictlimit" feature was introduced to enforce per-bdi dirty limits for
> FUSE which sets bdi max_ratio to 1% by default:
> 
> http://www.http.com//article.gmane.org/gmane.linux.kernel.mm/105809
> 
> However the feature can be useful for other relatively slow or untrusted
> BDIs like USB flash drives and DVD+RW. The patch adds a knob to enable the
> feature:
> 
> echo 1 > /sys/class/bdi/X:Y/strictlimit
> 
> Being enabled, the feature enforces bdi max_ratio limit even if global (10%)
> dirty limit is not reached. Of course, the effect is not visible until
> max_ratio is decreased to some reasonable value.

I suggest replacing "max_ratio" here with the much more informative
"/sys/class/bdi/X:Y/max_ratio".

Also, Documentation/ABI/testing/sysfs-class-bdi will need an update
please.

>  mm/backing-dev.c |   35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 

I'm not really sure what to make of the patch.  I assume you tested it
and observed some effect.  Could you please describe the test setup and
the effects in some detail?

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