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Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN
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From:   John Hubbard <jhubbard@nvidia.com>
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On 11/4/19 1:15 PM, Jason Gunthorpe wrote:
...
>> Right, and I thought about this when converting, and realized that the above 
>> code is working around the current gup.c limitations, which are "cannot support
>> gup remote with FOLL_LONGTERM".
> 
> But AFAICT it doesn't have a problem, the protection test is just too
> strict, and I guess the control flow needs a bit of fixing..
> 
> The issue is this:
> 
> static __always_inline long __get_user_pages_locked():
> {
>         if (locked) {
>                 /* if VM_FAULT_RETRY can be returned, vmas become invalid */
>                 BUG_ON(vmas);
>                 /* check caller initialized locked */
>                 BUG_ON(*locked != 1);
>         }
> 
> 
> so remote could be written as:
> 
> if (gup_flags & FOLL_LONGTERM) {
>    if (WARN_ON_ONCE(locked))
>         return -EINVAL;
>    return __gup_longterm_locked(...)
> }
> 
> return __get_user_pages_locked(...)
> 
> ??

Yes, that loosens it up just enough for the vfio case (which doesn't set 
"locked") to get through, great! OK, I'll put that (the above plus 
corresponding vfio fix) in a separate patch first. 

This should clear things up nicely.


thanks,
-- 
John Hubbard
NVIDIA