On Thu, 8 Sept 2022 at 17:36, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > Hi Alessio and Miklos, > > Some time has passed.. and I was thinking of picking up these patches. > > On Mon, Mar 1, 2021 at 7:05 PM Alessio Balsini <balsini@xxxxxxxxxxx> wrote: > > > > On Fri, Feb 19, 2021 at 09:40:21AM +0100, Miklos Szeredi wrote: > > > On Fri, Feb 19, 2021 at 8:05 AM Peng Tao <bergwolf@xxxxxxxxx> wrote: > > > > > > > > On Wed, Feb 17, 2021 at 9:41 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > > > > > > > What I think would be useful is to have an explicit > > > > > FUSE_DEV_IOC_PASSTHROUGH_CLOSE ioctl, that would need to be called > > > > > once the fuse server no longer needs this ID. If this turns out to > > > > > be a performance problem, we could still add the auto-close behavior > > > > > with an explicit FOPEN_PASSTHROUGH_AUTOCLOSE flag later. > > > > Hi Miklos, > > > > > > > > W/o auto closing, what happens if user space daemon forgets to call > > > > FUSE_DEV_IOC_PASSTHROUGH_CLOSE? Do we keep the ID alive somewhere? > > > > > > Kernel would keep the ID open until explicit close or fuse connection > > > is released. > > > > > > There should be some limit on the max open files referenced through > > > ID's, though. E.g. inherit RLIMIT_NOFILE from mounting task. > > > > > > Thanks, > > > Miklos > > > > I like the idea of FUSE_DEV_IOC_PASSTHROUGH_CLOSE to revoke the > > passthrough access, that is something I was already working on. What I > > had in mind was simply to break that 1:1 connection between fuse_file > > and lower filp setting a specific fuse_file::passthrough::filp to NULL, > > but this is slightly different from what you mentioned. > > > > I don't like the idea of switching between passthrough and server mid-life > of an open file. > > There are consequences related to syncing the attribute cache of the kernel > and the server that I don't even want to think about. > > > AFAIU you are suggesting to allocate one ID for each lower fs file > > opened with passthrough within a connection, and maybe using idr_find at > > every read/write/mmap operation to check if passthrough is enabled on > > that file. Something similar to fuse2_map_get(). > > This way the fuse server can pass the same ID to one or more > > fuse_file(s). > > FUSE_DEV_IOC_PASSTHROUGH_CLOSE would idr_remove the ID, so idr_find > > would fail, preventing the use of passthrough on that ID. CMIIW. > > > > I don't think that FUSE_DEV_IOC_PASSTHROUGH_CLOSE should remove the ID. > We can use a refcount for the mapping and FUSE_DEV_IOC_PASSTHROUGH_CLOSE > just drops the initial server's refcount. > > Implementing revoke for an existing mapping is something completely different. > It can be done, not even so hard, but I don't think it should be part of this > series and in any case revoke will not remove the ID. > > > After FUSE_DEV_IOC_PASSTHROUGH_CLOSE(ID) it may happen that if some > > fuse_file(s) storing that ID are still open and the same ID is reclaimed > > in a new idr_alloc, this would lead to mismatching lower fs filp being > > used by our fuse_file(s). So also the ID stored in the fuse_file(s) > > must be invalidated to prevent future uses of deallocated IDs. > > Obtaining a refcount on FOPEN_PASSTHROUGH will solve that. > > > > > Would it make sense to have a list of fuse_files using the same ID, that > > must be traversed at FUSE_DEV_IOC_PASSTHROUGH_CLOSE time? > > Negative values (maybe -ENOENT) might be used to mark IDs as invalid, > > and tested before idr_find at read/write/mmap to avoid the idr_find > > complexity in case passthrough is disabled for that file. > > > > What do you think? > > > > As I wrote above, this sounds unnecessarily complicated. > > Miklos, > > Do you agree with my interpretation of > FUSE_DEV_IOC_PASSTHROUGH_CLOSE? We need to deal with the case of too many open files. The server could manage this, but then we do need to handle the case when a cached mapping disappears, i.e: client opens file [time passes] cached passthrough fd gets evicted to make room for other passthrough I/O [time passes] new I/O request comes in need to reestablish passthrough fd before finishing I/O The way I think of this is that a passthrough mapping is assigned at open time, which is cached (which may have the lifetime longer than the open file, but shorter as well). When FUSE_DEV_IOC_PASSTHROUGH_CLOSE and there are cached mapping referring to this particular handle, then those mappings need to be purged. On a new I/O request, the mapping will need to be reestablished by sending a FUSE_MAP request, which triggers FUSE_DEV_IOC_PASSTHROUGH_OPEN. One other question that's nagging me is how to "unhide" these pseudo-fds. Could we create a kernel thread for each fuse sb which has normal file-table for these? This would would allow inspecting state through /proc/$KTHREDID/fd, etc.. Thanks, Miklos
