Re: [RESEND 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

[Jason Gunthorpe]

On Mon, Mar 25, 2019 at 02:23:15AM -0700, Ira Weiny wrote:
> > > Unfortunately holding the lock is required to support FOLL_LONGTERM (to check
> > > the VMAs) but we don't want to hold the lock to be optimal (specifically allow
> > > FAULT_FOLL_ALLOW_RETRY).  So I'm maintaining the optimization for *_fast users
> > > who do not specify FOLL_LONGTERM.
> > > 
> > > Another way to do this would have been to define __gup_longterm_unlocked with
> > > the above logic, but that seemed overkill at this point.
> > 
> > get_user_pages_unlocked() is an exported symbol, shouldn't it work
> > with the FOLL_LONGTERM flag?
> > 
> > I think it should even though we have no user..
> > 
> > Otherwise the GUP API just gets more confusing.
> 
> I agree WRT to the API.  But I think callers of get_user_pages_unlocked() are
> not going to get the behavior they want if they specify FOLL_LONGTERM.

Oh? Isn't the only thing FOLL_LONGTERM does is block the call on DAX?
Why does the locking mode matter to this test?

> What I could do is BUG_ON (or just WARN_ON) if unlocked is called with
> FOLL_LONGTERM similar to the code in get_user_pages_locked() which does not
> allow locked and vmas to be passed together:

The GUP call should fail if you are doing something like this. But I'd
rather not see confusing specialc cases in code without a clear
comment explaining why it has to be there.

Jason