Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches

[John Hubbard]

On 5/1/24 11:56 PM, David Hildenbrand wrote:
> On 02.05.24 03:05, Alistair Popple wrote:
> > Jason Gunthorpe <jgg@xxxxxxxxxx> writes:
..
> > > > This doesn't make sense.  IFF a blind retry is all that is needed it
> > > > should be done in the core functionality.  I fear it's not that easy,
> > > > though.
> > >
> > > +1
> > >
> > > This migration retry weirdness is a GUP issue, it needs to be solved
> > > in the mm not exposed to every pin_user_pages caller.
> > >
> > > If it turns out ZONE_MOVEABLE pages can't actually be reliably moved
> > > then it is pretty broken..
> >
> > I wonder if we should remove the arbitrary retry limit in
> > migrate_pages() entirely for ZONE_MOVEABLE pages and just loop until
> > they migrate? By definition there should only be transient references on
> > these pages so why do we need to limit the number of retries in the
> > first place?
>
> There are some weird things that still needs fixing: vmsplice() is the 
> example that doesn't use FOLL_LONGTERM.

Hi David!

Do you have any other call sites in mind? It sounds like one way forward
is to fix each call site...

This is an unhappy story right now: the pin_user_pages*() APIs are
significantly worse than before the "migrate pages away automatically"
upgrade, from a user point of view. Because now, the APIs fail
intermittently for callers who follow the rules--because
pin_user_pages() is not fully working yet, basically.

Other ideas, large or small, about how to approach a fix?

thanks,

--
John Hubbard
NVIDIA