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

David Hildenbrand
2024-05-02T06:56:33Z

On 02.05.24 03:05, Alistair Popple wrote:
>
> Jason Gunthorpe <jgg@nvidia.com> writes:
>
>> 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..
>
> 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.

--
Cheers,

David / dhildenb