CC fuse maintainer, too.
On Wed, Dec 06, 2017 at 05:09:27PM -0800, Andrew Morton wrote:
On Fri, 1 Dec 2017 13:29:28 +0100 Jan Kara <jack@xxxxxxx> wrote:
On Thu 30-11-17 14:15:58, Andrew Morton wrote:
> From: Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx>
> 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@xxxxxxx>
Cc Jens, who may be vaguely interested in plans to finally merge this
three-year-old patch?
From: Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx>
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
That link is invalid for now, possibly due to the gmane site rebuild.
I find an email thread here which looks relevant:
https://sourceforge.net/p/fuse/mailman/message/35254883/
Where Maxim has an interesting point:
> Did any one try increasing the limit and did see any better/worse
> performance ?
We've used 20% as default value in OpenVZ kernel for a long while (1%
was not enough to saturate our distributed parallel storage).
So the knob will also enable people to _disable_ the 1% fuse limit to
increase performance.
So people can use the exposed knob in 2 ways to fit their needs, which
is in general a good thing.
However the comment in wb_position_ratio() says
Without strictlimit feature, fuse writeback may
* consume arbitrary amount of RAM because it is accounted in
* NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
How dangerous would that be if some user disabled the 1% fuse limit
through the exposed knob? Will the NR_WRITEBACK_TEMP effect go far
beyond the user's expectation (20% max dirty limit)?
Looking at the fuse code, NR_WRITEBACK_TEMP will grow proportional to
WB_WRITEBACK, which should be throttled when bdi_write_congested().
The congested flag will be set on
fuse_conn.num_background >= fuse_conn.congestion_threshold
So it looks NR_WRITEBACK_TEMP will somehow be throttled. Just that
it's not included in the 20% dirty limit.
Other than that concern, the patch looks good to me.
Thanks,
Fengguang
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.
Jan said:
: 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:
Signed-off-by: Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Henrique de Moraes Holschuh <hmh@xxxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: "Artem S. Tashkinov" <t.artem@xxxxxxxxx>
Cc: Mel Gorman <mel@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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
@@ -231,11 +231,46 @@ 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);
_
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