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

David Hildenbrand
2024-05-10T07:54:28Z

On 02.05.24 20:10, John Hubbard wrote:
> On 5/1/24 11:56 PM, David Hildenbrand wrote:
>> On 02.05.24 03:05, Alistair Popple wrote:
>>> Jason Gunthorpe <jgg@nvidia.com> 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!
>

Sorry for the late reply!

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

We also have udmabuf, that is currently getting fixed [1] similarly to
how we handle GUP. Could you and/or Jason also have a look at the
GUP-related bits? I acked it but the patch set does not seem to make
progress.

https://lkml.kernel.org/r/20240411070157.3318425-1-vivek.kasireddy@intel.com

The sad story is:

(a) vmsplice() is harder to fix (identify all put_page() and replace
them by unpin_user_page()), but will get fixed at some point.

(b) even !longterm DMA can take double-digit seconds

(c) other wrong code is likely to exist or to appear again and it's
rather hard to identify+prevent reliably

IMHO we should expect migration to take a longer time and maybe never
happening in some cases.

Memory offlining (e.g., echo "offline" >
sys/devices/system/memory/memory0/state) currently tries forever to
migrate pages and can be killed if started from user space using a fatal
signal. If memory offlining happens from kernel context (unplugging
DIMM, ACPI code triggers offlining), we'd much rather want to fail at
some point instead of looping forever, but it hasn't really popped up as
a problem so far.

virtio-mem uses alloc_contig_range() for best-effort allocation and will
skip such temporarily unmovable ranges to try again later. Here, we
really don't want to loop forever in migration code but rather fail
earlier and try unplug of another memory block.

So as long as page pinning is triggered from user context where the user
can simply abort the process (e.g., kill the process), sleep+retry on
ZONE_MOVABLE + MIGRATE_CMA sounds reasonable.

>
> 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?

What Jason says makes sense to me: sleep+retry. My only concern is when
pin_user_pages_*() is called from non-killable context where failing at
some point might be more reasonable. But maybe that use case doesn't
really exist?

--
Cheers,

David / dhildenb