On Wed, 26 Oct 2022 at 13:58, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Tue, 25 Oct 2022 at 17:39, Antonio SJ Musumeci <trapexit@xxxxxxxxxx> wrote: > > > > On 9/19/22 05:20, Miklos Szeredi wrote: > > > On Sun, 18 Sept 2022 at 13:03, Nikolaus Rath <Nikolaus@xxxxxxxx> wrote: > > >> Hi, > > >> > > >> Should the FUSE kernel driver perhaps set PR_SET_IO_FLUSHER for the FUSE > > >> userspace process daemon when a connection is opened? > > >> > > >> If I understand correctly, this is necessary to avoid a deadlocks if the > > >> kernel needs to reclaim memory that has to be written back through FUSE. > > > The fuse kernel driver is careful to avoid such deadlocks. When > > > memory reclaim happens, it copies data to temporary buffers and > > > immediately finishes the reclaim from the memory management > > > subsystem's point of view. The temp buffers are then sent to > > > userspace and written back without having to worry about deadlocks. > > > There are lots of details missing from the above description, but this > > > is the essence of the writeback deadlock avoidance. > > > > > > Thanks, > > > Miklos > > > > Miklos, does this mean that FUSE servers shouldn't bother setting > > PR_SET_IO_FLUSHER? Are there any benefits to setting it explicitly or > > detriments to not setting it? > > PR_SET_IO_FLUHSER internally sets the process flags PF_MEMALLOC_NOIO > and PF_LOCAL_THROTTLE. > > The former is clear: don't try to initiate I/O when memory needs to be > reclaimed. This could be detrimental in low memory situations, since > the kernel has less choice for freeing up memory. > > PF_LOCAL_THROTTLE seems to mean "don't throttle dirtying pages > (writes) by this process, since that would throttle the cleaning of > dirty pages." This logic seems valid for fuse as well, but it also > upsets the normal dirty throttling mechanisms, so I'm not sure that > there aren't any side effects. Also consider: when a fuse page is under writeback, it's already accounted as "clean" for the purposes reclaim and for throttling other dirtiers. Throttling of fuse fuse writeback pages (NR_WRITEBACK_TEMP) is done completely separately. So I'd say, it's better not to set PR_SET_IO_FLUSHER on the fuse daemon, although there probably wouldn't be any catastrophic consequences of setting it. Thanks, Miklos
