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

Jason Gunthorpe
2024-05-01T12:10:39Z

On Tue, Apr 30, 2024 at 10:10:43PM -0700, Christoph Hellwig wrote:
> > + pinned = -ENOMEM;
> > + int attempts = 0;
> > + /*
> > + * pin_user_pages_fast() can return -EAGAIN, due to falling back
> > + * to gup-slow and then failing to migrate pages out of
> > + * ZONE_MOVABLE due to a transient elevated page refcount.
> > + *
> > + * One retry is enough to avoid this problem, so far, but let's
> > + * use a slightly higher retry count just in case even larger
> > + * systems have a longer-lasting transient refcount problem.
> > + *
> > + */
> > + static const int MAX_ATTEMPTS = 3;
> > +
> > + while (pinned == -EAGAIN && attempts < MAX_ATTEMPTS) {
> > + pinned = pin_user_pages_fast(cur_base,
> > + min_t(unsigned long,
> > + npages, PAGE_SIZE /
> > + sizeof(struct page *)),
> > + gup_flags, page_list);
> > ret = pinned;
> > - goto umem_release;
> > + attempts++;
> > +
> > + if (pinned == -EAGAIN)
> > + continue;
> > }
> > + if (pinned < 0)
> > + goto umem_release;
>
> 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..

Jason