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

From: John Hubbard
Date: Mon Feb 11 2019 - 16:22:18 EST

On 2/11/19 10:19 AM, Ira Weiny wrote:
> On Mon, Feb 11, 2019 at 11:06:54AM -0700, Jason Gunthorpe wrote:
>> On Mon, Feb 11, 2019 at 09:22:58AM -0800, Dan Williams wrote:
> John's patches will indicate to the FS that the page is gup pinned. But they
> will not indicate longterm vs not "shorterm". A shortterm pin could be handled
> as a "real truncate". So, are we back to needing a longterm "bit" in struct
> page to indicate a longterm pin and allow the FS to perform this "virtual
> write" after truncate?
> Or is it safe to consider all gup pinned pages this way?
> Ira

I mentioned this in another thread, but I'm not great at email threading. :)
Anyway, it seems better to just drop the entire "longterm" concept from the
internal APIs, and just deal in "it's either gup-pinned *at the moment*, or
it's not". And let the filesystem respond appropriately. So for a pinned page
that hits clear_page_dirty_for_io or whatever else care about pinned pages:

-- fire mmu notifiers, revoke leases, generally do everything as if it were a
long term gup pin

-- if it's long term, then you've taken the right actions.

-- if the pin really is short term, everything works great anyway.

The only way that breaks is if longterm pins imply an irreversible action, such
as blocking and waiting in a way that you can't back out of or get interrupted
out of. And the design doesn't seem to be going in that direction, right?

John Hubbard