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

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On 18.05.23 08:08, Sumit Garg wrote:
On Thu, 18 May 2023 at 09:51, Christoph Hellwig <hch@xxxxxxxxxxxxx> 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





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