Re: reply: [RFC] pin_user_pages_fast failure count increased

[David Hildenbrand <david@redhat.com>]

On 28.05.25 05:36, Hyesoo Yu wrote:
> On Wed, May 28, 2025 at 10:49:36AM +0800, Zhaoyang Huang wrote:
>> On Wed, May 28, 2025 at 9:25 AM Hyesoo Yu <hyesoo.yu@samsung.com> wrote:
>>>
>>> On Mon, May 26, 2025 at 07:49:57PM +0800, Zhaoyang Huang wrote:
>>>
>>> Hello, Zhaoyang.
>>>
>>> I don't believe commit 1aaf8c was just intended to prevent an infinite loop.
>>> The commit was introduced to allow pinning CMA memory in the pKVM on AOSP.
>>>
>>> That leads me to question whether the assumption that CMA can be long-term pinned is actually valid.
>> That depends on the user of CMA, yes for my scenario since it worked
>> for the guest os. For common scenario such as the file/anon mapping,
>> the page will be judged as unpinnable for long-term and be migrated
>> out of CMA area.
> 
> Your scenario and the common scenarios can not be distinguished from the kernel API's perspective.
> Even in common cases, the page may be in a non-LRU state temporiarily, and in such situations,
> pinning CMA can lead to bugs - we've encountered multiple issues because of this.
> 

Right. We just disallow long-term pinning CMA pages, because we don't 
know who the real owner is that would be okay with long-term pinning them.

>>>
>>> In my opinion, it might be more appropriate to revert that commit 1aaf8c and instead ensure
>>> that pKVM avoids using CMA for memory that requires long-term pinning through GUP ?
>> It is not a pkvm issue but a defect of applying FOLL_LONGTERM over
>> non-LRU CMA pages.
> 
> In include/linux/mm_types.h, the CMA should be migrated when FOLL_LONGTERM.
> 
> * In the CMA case: long term pins in a CMA region would unnecessarily fragment
> * that region.  And so, CMA attempts to migrate the page before pinning, when
> * FOLL_LONGTERM is specified.
> 
> Given this, would it make sense to avoid using FOLL_LONGTERM in this code path ?

If something is unbounded in time, FOLL_LONGTERM is the right thing to use.

>>>
>>> Alternatively, instead of changing the current logic that prevents longterm GUP from pinning CMA,
>>> it would be better to propose a new patch that specifically addresses the pKVM scenario like adding new FOLL_flags ?
>> I don't think so. pin_user_pages is an exported API which can't make
>> assumptions over the caller.
> 
> My point is not to base the patch on assumptions about the caller,
> but to define a clear mechanism that ensures safe behavior in the intended scenario.
> 
> For example, you can add FOLL_NO_MIGRATION and skip to migrate unpinnable pages.

Not sure which exact semantics you have in mind. But failing if we would 
have to migrate might be ok. Not sure if the caller should worry about 
that, though: the caller should not have to worry about page placement 
in general.

-- 
Cheers,

David / dhildenb