On Tue, 6 Jun 2023 at 13:00, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > On Tue, Jun 6, 2023 at 1:23 PM Miklos Szeredi via fuse-devel > <fuse-devel@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Fri, 19 May 2023 at 14:57, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > > > WIP > > > > > > Add an ioctl to associate a FUSE server open fd with a request. > > > A later response to this request get use the FOPEN_PASSTHROUGH flag > > > to request passthrough to the associated backing file. > > > > > > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx> > > > --- > > > > > > Miklos, > > > > > > After implementing refcounted backing files, I started to think how > > > to limit the server from mapping too many files. > > > > > > I wanted to limit the backing files mappings to the number of open fuse > > > files to simplify backing files accounting (i.e. open files are > > > effectively accounted to clients). > > > > > > It occured to me that creatig a 1-to-1 mapping between fuse files and > > > backing file ids is quite futile if there is no need to manage 1-to-many > > > backing file mappings. > > > > > > If only 1-to-1 mapping is desired, the proposed ioctl associates a > > > backing file with a pending request. The backing file will be kept > > > open for as long the request lives, or until its refcount is handed > > > over to the client, which can then use it to setup passthough to the > > > backing file without the intermediate idr array. > > > > I think I understand what the patch does, but what I don't understand > > is how this is going to solve the resource accounting problem. > > > > Can you elaborate? > > > > It does not solve the resource accounting in the traditional way > of limiting the number of open files to the resource limit of the > server process. > > Instead, it has the similar effect of overlayfs pseudo files > non accounting. > > A FUSE passthrough filesystem can contribute the same number > of non accounted open fds as the number of FUSE fds accounted > to different processes. > > A non privileged user can indirectly cause unaccounted open fds > with a FUSE passthough fs in the exact same way that the same > user can cause unaccounted open fds with an overlayfs mount > if it can convince other users to open files on the FUSE/ovl that it > has mounted. > > Am I making sense? So this allows double the number of open files as normally would be allowed, same as overlayfs. Makes sense. Thanks, Miklos
