Re: reply: [RFC] pin_user_pages_fast failure count increased

[David Hildenbrand <david@redhat.com>]

>>>>>
>>>> So, what's the status of that? We should fix it upstream (*not* caring
>>>> about controversial out-of-tree pkvm issues).
>>> Leaving aside the pkvm issue, we should also care about the CMA pages
>>> mapping to VM by special driver which are intended to be long term
>>> pinned (actually they are fetched by cma_alloc and then mapped to VM
>>> instead of alloc_pages during normal page fault).
>>
>> Is there any such "special driver" in the tree?
> Not that I know of. However, pin_user_pages is exported symbol which
> could be used for ko, should we make it be capable of dealing with
> this scenario?

We have the tendency to not go above and beyond of adding features that 
we cannot even test without OOT drivers.

Note that we export symbols so other in-tree drivers that are built as a 
module can make use of them.

>>
>>> Could we distinguish
>>> them by the patch below based on 1aaf8c122, that is, this kind of
>>> pages is not on page cache and have equaled refcnt to mapcount
>>
>> No, not like that. We'd need some proper indication that this page was
>> allocated by the CMA area owner, and that owner agrees that the folio
>> can be long-term pinned (maybe that agreement is by mapping it into user
>> space, tbd).
> I think the key point is to distinguish the cma pages which are
> allocated from fallback of GFP_MOVABLE during common page faults from
> the ones which got from cma_alloc within the special driver's
> vm_ops->fault.

Yes. Using typed pages in the future might work. For now, this is not 
possible yet because the page type overlays page->mapcount.

Hm.

>>
>> Will you send the fix or should I do it? Discussing about broken use
>> cases that do no apply upstream is not particularly helpful when we're
>> dealing with a real upstream bug.
> I would like to ask for your help on this since I have no further ideas. Thanks

Okay, let me send a fix for the original commit.

-- 
Cheers,

David / dhildenb