On Thu, Dec 7, 2017 at 5:14 AM, Fengguang Wu <fengguang.wu@xxxxxxxxx> wrote: > 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. Only balance_dirty_pages_ratelimited() is going to limit the generation of dirty pages, I don't think congestion flags will do that. And (AFAICS) for fuse only BDI_CAP_STRICTLIMIT will allow accounting temp writeback pages when throttling dirty page generation. So without BDI_CAP_STRICTLIMIT kernel memory use of fuse may explode. So we probably need a way to force BDI_CAP_STRICTLIMIT (i.e. do not permit disabling it for fuse). Please correct me if I'm wrong in any of the above statements, it's been a long time I've taken a detailed look at the page writeback mechanisms. Thanks, Miklos
