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. Thanks, Miklos
