Re: [PATCH v9 0/6] MAP_DIRECT for DAX userspace flush

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Oct 13, 2017 at 9:38 AM, Jason Gunthorpe
<jgunthorpe@obsidianresearch.com> wrote:
> On Fri, Oct 13, 2017 at 08:14:55AM -0700, Dan Williams wrote:
>
>> scheme specific to RDMA which seems like a waste to me when we can
>> generically signal an event on the fd for any event that effects any
>> of the vma's on the file. The FL_LAYOUT lease impacts the entire file,
>> so as far as I can see delaying the notification until MR-init is too
>> late, too granular, and too RDMA specific.
>
> But for RDMA a FD is not what we care about - we want the MR handle so
> the app knows which MR needs fixing.

I'd rather put the onus on userspace to remember where it used a
MAP_DIRECT mapping and be aware that all the mappings of that file are
subject to a lease break. Sure, we could build up a pile of kernel
infrastructure to notify on a per-MR basis, but I think that would
only be worth it if leases were range based. As it is, the entire file
is covered by a lease instance and all MRs that might reference that
file get one notification. That said, we can always arrange for a
per-driver callback at lease-break time so that it can do something
above and beyond the default notification.
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