On Wed, Feb 06, 2019 at 01:32:04PM -0500, Doug Ledford wrote: > On Wed, 2019-02-06 at 09:52 -0800, Matthew Wilcox wrote: > > On Wed, Feb 06, 2019 at 10:31:14AM -0700, Jason Gunthorpe wrote: > > > On Wed, Feb 06, 2019 at 10:50:00AM +0100, Jan Kara wrote: > > > > > > > MM/FS asks for lease to be revoked. The revoke handler agrees with the > > > > other side on cancelling RDMA or whatever and drops the page pins. > > > > > > This takes a trip through userspace since the communication protocol > > > is entirely managed in userspace. > > > > > > Most existing communication protocols don't have a 'cancel operation'. > > > > > > > Now I understand there can be HW / communication failures etc. in > > > > which case the driver could either block waiting or make sure future > > > > IO will fail and drop the pins. > > > > > > We can always rip things away from the userspace.. However.. > > > > > > > But under normal conditions there should be a way to revoke the > > > > access. And if the HW/driver cannot support this, then don't let it > > > > anywhere near DAX filesystem. > > > > > > I think the general observation is that people who want to do DAX & > > > RDMA want it to actually work, without data corruption, random process > > > kills or random communication failures. > > > > > > Really, few users would actually want to run in a system where revoke > > > can be triggered. > > > > > > So.. how can the FS/MM side provide a guarantee to the user that > > > revoke won't happen under a certain system design? > > > > Most of the cases we want revoke for are things like truncate(). > > Shouldn't happen with a sane system, but we're trying to avoid users > > doing awful things like being able to DMA to pages that are now part of > > a different file. > > Why is the solution revoke then? Is there something besides truncate > that we have to worry about? I ask because EBUSY is not currently > listed as a return value of truncate, so extending the API to include > EBUSY to mean "this file has pinned pages that can not be freed" is not > (or should not be) totally out of the question. > > Admittedly, I'm coming in late to this conversation, but did I miss the > portion where that alternative was ruled out? That's my preferred option too, but the preponderance of opinion leans towards "We can't give people a way to make files un-truncatable".
