Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone

[David Hildenbrand]

> Am 11.12.2020 um 22:36 schrieb Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>:
> 
> On Fri, Dec 11, 2020 at 4:29 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>> 
>> 
>>>> Am 11.12.2020 um 22:09 schrieb Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>:
>>> 
>>> On Fri, Dec 11, 2020 at 3:46 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>>>> 
>>>>> On Fri, Dec 11, 2020 at 03:40:57PM -0500, Pavel Tatashin wrote:
>>>>> On Fri, Dec 11, 2020 at 3:23 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>>>>>> 
>>>>>> On Fri, Dec 11, 2020 at 03:21:39PM -0500, Pavel Tatashin wrote:
>>>>>>> @@ -1593,7 +1592,7 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
>>>>>>>                             }
>>>>>>> 
>>>>>>>                             if (!isolate_lru_page(head)) {
>>>>>>> -                                     list_add_tail(&head->lru, &cma_page_list);
>>>>>>> +                                     list_add_tail(&head->lru, &movable_page_list);
>>>>>>>                                     mod_node_page_state(page_pgdat(head),
>>>>>>>                                                         NR_ISOLATED_ANON +
>>>>>>>                                                         page_is_file_lru(head),
>>>>>>> @@ -1605,7 +1604,7 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
>>>>>>>             i += step;
>>>>>>>     }
>>>>>>> 
>>>>>>> -     if (!list_empty(&cma_page_list)) {
>>>>>>> +     if (!list_empty(&movable_page_list)) {
>>>>>> 
>>>>>> You didn't answer my earlier question, is it OK that ZONE_MOVABLE
>>>>>> pages leak out here if ioslate_lru_page() fails but the
>>>>>> moval_page_list is empty?
>>>>>> 
>>>>>> I think the answer is no, right?
>>>>> In my opinion it is OK. We are doing our best to not pin movable
>>>>> pages, but if isolate_lru_page() fails because pages are currently
>>>>> locked by someone else, we will end up long-term pinning them.
>>>>> See comment in this patch:
>>>>> +        * 1. Pinned pages: (long-term) pinning of movable pages is avoided
>>>>> +        *    when pages are pinned and faulted, but it is still possible that
>>>>> +        *    address space already has pages in ZONE_MOVABLE at the time when
>>>>> +        *    pages are pinned (i.e. user has touches that memory before
>>>>> +        *    pinning). In such case we try to migrate them to a different zone,
>>>>> +        *    but if migration fails the pages can still end-up pinned in
>>>>> +        *    ZONE_MOVABLE. In such case, memory offlining might retry a long
>>>>> +        *    time and will only succeed once user application unpins pages.
>>>> 
>>>> It is not "retry a long time" it is "might never complete" because
>>>> userspace will hold the DMA pin indefinitely.
>>>> 
>>>> Confused what the point of all this is then ??
>>>> 
>>>> I thought to goal here is to make memory unplug reliable, if you leave
>>>> a hole like this then any hostile userspace can block it forever.
>>> 
>>> You are right, I used a wording from the previous comment, and it
>>> should be made clear that pin may be forever. Without these patches it
>>> is guaranteed that hot-remove will fail if there are pinned pages as
>>> ZONE_MOVABLE is actually the first to be searched. Now, it will fail
>>> only due to exceptions listed in ZONE_MOVABLE comment:
>>> 
>>> 1. pin + migration/isolation failure
>> 
>> Not sure what that really means. We have short-term pinnings (although we might have a better term for „pinning“ here) for example, when a process dies (IIRC). There is a period where pages cannot get migrated and offlining code has to retry (which might take a while). This still applies after your change - are you referring to that?
>> 
>>> 2. memblock allocation due to limited amount of space for kernelcore
>>> 3. memory holes
>>> 4. hwpoison
>>> 5. Unmovable PG_offline pages (? need to study why this is a scenario).
>> 
>> Virtio-mem is the primary user in this context.
>> 
>>> Do you think we should unconditionally unpin pages, and return error
>>> when isolation/migration fails?
>> 
>> I‘m not sure what you mean here. Who’s supposed to unpin which pages?
> 
> Hi David,
> 
> When check_and_migrate_movable_pages() is called, the pages are
> already pinned. If some of those pages are in movable zone, and we
> fail to migrate or isolate them what should we do: proceed, and keep
> it as exception of when movable zone can actually have pinned pages or
> unpin all pages in the array, and return an error, or unpin only pages
> in movable zone, and return an error?
> 

I guess revert what we did (unpin) and return an error. The interesting question is what can make migration/isolation fail

a) out of memory: smells like a zone setup issue. Failures are acceptable I guess.

b) short term pinnings: process dying - not relevant I guess. Other cases? (Fork?)

c) ?

Once we clarified that, we actually know how likely it will be to return an error (and making vfio pinnings fail etc).

> Pasha