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
