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

Christoph Hellwig
2024-05-01T05:10:44Z

> + 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.