Re: set_page_dirty vs set_page_dirty_lock

[Nick Piggin <nickpiggin@yahoo.com.au>]

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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