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 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); _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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