FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm

[David Hildenbrand]

On 18.05.23 08:08, Sumit Garg wrote:
> On Thu, 18 May 2023 at 09:51, Christoph Hellwig <hch at infradead.org> wrote:
>>
>> On Wed, May 17, 2023 at 08:23:33PM +0200, David Hildenbrand wrote:
>>> In general: if user space controls it -> possibly forever -> long-term. Even
>>> if in most cases it's a short delay: there is no trusting on user space.
>>>
>>> For example, iouring fixed buffers keep pages pinned until user space
>>> decides to unregistered the buffers -> long-term.
>>>
>>> Short-term is, for example, something like O_DIRECT where we pin -> DMA ->
>>> unpin in essentially one operation.
>>
>> Btw, one thing that's been on my mind is that I think we got the
>> polarity on FOLL_LONGTERM wrong.  Instead of opting into the long term
>> behavior it really should be the default, with a FOLL_EPHEMERAL flag
>> to opt out of it.  And every users of this flag is required to have
>> a comment explaining the life time rules for the pin..
> 
> It does look like a better approach to me given the very nature of
> user space pages.

Yeah, there is a lot of historical baggage. For example, FOLL_GET should 
be inaccessible to kernel modules completely at one point, to be only 
used by selected core-mm pieces.

Maybe we should even disallow passing in FOLL_LONGTERM as a flag and 
only provide functions like pin_user_pages() vs. 
pin_user_pages_longterm(). Then, discussions about conditional 
flag-setting are no more :)

... or even use pin_user_pages_shortterm() vs. pin_user_pages() ... to 
make the default be longterm.

-- 
Thanks,

David / dhildenb