Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

[Dan Williams]

On Wed, Feb 6, 2019 at 10:52 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Wed, Feb 06, 2019 at 10:35:04AM -0800, Matthew Wilcox wrote:
>
> > > 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".
>
> I haven't heard an explanation why blocking ftruncate is worse than
> giving people a way to break RDMA using process by calling ftruncate??
>
> Isn't it exactly the same argument the other way?

No, I don't think it is. The lease is there to set the expectation of
getting out of the way, it's not a silent un-coordinated failure. The
user asked for it, the kernel is just honoring a valid request. If the
RDMA application doesn't want it to happen, arrange for it by
permissions or other coordination to prevent truncation, but once the
two conflicting / valid requests have arrived at the filesystem try to
move the result forward to the user requested state not block and fail
indefinitely.