Michael S. Tsirkin wrote:
FWIW, I think that copy_to_user will work correctly since it keeps the mmap
semaphore for the duration of the copy.
Oh, true.
Direct-io might have the same problem.
As such, I don't think it would be something you in particular need to
worry about.
I guess to solve it, we could either retain mmap_sem for the duration to
prevent fork,
Since this is the receive side, the DMA can take an indefinite
time to arrive. Isnt this a problem if we keep the mmap_sem?
Well... it goes against our usual stance of trying to push
these kinds of synchronisation issues to userspace.
In a way, you're doing direct-io from the network and so it is
perhaps reasonable to expect the racy semantics that O_DIRECT
has. OTOH, if you are providing the same API on a different
device, then basically by definition you need to provide the
exact same semantics.
or try to do something tricky with page_count to determine
if we need to do a copy in fork() rather than a COW.
I'm actually reasonably happy with the trick that I'm using:
performing a second get_user_pages after DMA and comparing
the page lists.
However, doing this every time on the off chance that a
page was made COW forces me into task context, every time.
I think it might be possible to solve it with the early-copy in
fork(). I'll tinker with it.
--
SUSE Labs, Novell Inc.
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