Re: [patch 15/15] mm: add strictlimit knob

[Jan Kara <jack@suse.cz>]

On Thu 30-11-17 14:15:58, Andrew Morton wrote:
> From: Maxim Patlasov <MPatlasov@parallels.com>
> Subject: mm: add strictlimit knob
> 
> The "strictlimit" feature was introduced to enforce per-bdi dirty limits
> for FUSE which sets bdi max_ratio to 1% by default:
> 
> http://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 /sys/class/bdi/X:Y/max_ratio is decreased to some reasonable value.

In principle I have nothing against this and the usecase sounds reasonable
(in fact I believe the lack of a feature like this is one of reasons why
desktop automounters usually mount USB devices with 'sync' mount option).
So feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
> Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: "Artem S. Tashkinov" <t.artem@lycos.com>
> Cc: Mel Gorman <mel@csn.ul.ie>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  Documentation/ABI/testing/sysfs-class-bdi |    8 ++++
>  mm/backing-dev.c                          |   35 ++++++++++++++++++++
>  2 files changed, 43 insertions(+)
> 
> diff -puN Documentation/ABI/testing/sysfs-class-bdi~mm-add-strictlimit-knob-v2 Documentation/ABI/testing/sysfs-class-bdi
> --- a/Documentation/ABI/testing/sysfs-class-bdi~mm-add-strictlimit-knob-v2
> +++ a/Documentation/ABI/testing/sysfs-class-bdi
> @@ -53,3 +53,11 @@ stable_pages_required (read-only)
>  
>  	If set, the backing device requires that all pages comprising a write
>  	request must not be changed until writeout is complete.
> +
> +strictlimit (read-write)
> +
> +	Forces per-BDI checks for the share of given device in the write-back
> +	cache even before the global background dirty limit is reached. This
> +	is useful in situations where the global limit is much higher than
> +	affordable for given relatively slow (or untrusted) device. Turning
> +	strictlimit on has no visible effect if max_ratio is equal to 100%.
> diff -puN mm/backing-dev.c~mm-add-strictlimit-knob-v2 mm/backing-dev.c
> --- a/mm/backing-dev.c~mm-add-strictlimit-knob-v2
> +++ a/mm/backing-dev.c
> @@ -218,11 +218,46 @@ static ssize_t stable_pages_required_sho
>  }
>  static DEVICE_ATTR_RO(stable_pages_required);
>  
> +static ssize_t strictlimit_store(struct device *dev,
> +		struct device_attribute *attr, const char *buf, size_t count)
> +{
> +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> +	unsigned int val;
> +	ssize_t ret;
> +
> +	ret = kstrtouint(buf, 10, &val);
> +	if (ret < 0)
> +		return ret;
> +
> +	switch (val) {
> +	case 0:
> +		bdi->capabilities &= ~BDI_CAP_STRICTLIMIT;
> +		break;
> +	case 1:
> +		bdi->capabilities |= BDI_CAP_STRICTLIMIT;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return count;
> +}
> +static ssize_t strictlimit_show(struct device *dev,
> +		struct device_attribute *attr, char *page)
> +{
> +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> +
> +	return snprintf(page, PAGE_SIZE-1, "%d\n",
> +			!!(bdi->capabilities & BDI_CAP_STRICTLIMIT));
> +}
> +static DEVICE_ATTR_RW(strictlimit);
> +
>  static struct attribute *bdi_dev_attrs[] = {
>  	&dev_attr_read_ahead_kb.attr,
>  	&dev_attr_min_ratio.attr,
>  	&dev_attr_max_ratio.attr,
>  	&dev_attr_stable_pages_required.attr,
> +	&dev_attr_strictlimit.attr,
>  	NULL,
>  };
>  ATTRIBUTE_GROUPS(bdi_dev);
> _
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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