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SubjectRe: get_user_pages question
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On Saturday 28 November 2009 20:50:52 you wrote:
> Hi Hugh and everyone,
>
> On Tue, Nov 10, 2009 at 04:33:30PM +0000, Hugh Dickins wrote:
> > In fairness I've added Andrea and KOSAKI-san to the Cc, since I know
> > they are two people keen to fix this issue once and for all. Whereas
>
> Right, I'm sure Nick also wants to fix this once and for all (adding
> him too to Cc ;).
>
> I thought and I still think it's bad to leave races like this open for
> people to find out the hard way. It just takes somebody to use
> pthread_create, open a file with O_DIRECT with 512byte (not page
> ....

Hello all!

First let me state that I solved my problems by simply avoiding GUP completely
and going with a clean mmap implemenation (with the nopage version) which
causes no problems what so ever. mmap does not suffer from all the problems
discussed above (aside from the fact that you have to do your own book keeping
as far as vma_open and vma_close and fault function goes...). Please correct
me if I'm wrong...:)

The fact that I solved all my problems with mmap and the complexity of the
proposed solutions got me thinking about GUP in more general terms. Would it
be fair to say that mmap is much more aligned to the kernels way of doing
things than GUP? It feels like the vma concept which is a solid one and
probably works well for most architectures is in conflict with GUP and so is
fork. It also feels like the vma concept is well aligned with fork which means
in turn that mmap is well aligned with fork while GUP is not. This is a new
conclusion for me and one which did not register back when reading the LDD
book (I got the impression that you can pick between mmap and GUP and it does
not really matter but now I feel that mmap is much advanced and trouble free).

Testing it out I grepped the drivers folder of a recent kernel with ONLY 26
mentions of GUP in the entire drivers folder! The main drivers using GUP are
scsi st and infiniband. If GUP is so unused is it really essential as an in
kernel interface? If GUP is slowly dropped and drivers converted to mmap would
it not simplify kernel code or at least prevent complex solutions to GUP
problems from complicating mm code even more? Again, I'm no kernel expert so
please don't flame me too hard if I'm talking heresy or nonsense, I would just
like to hear your take on this. It may well be that I simply have no clue and
so my conclusions are way too radical...

Cheers,
Mark


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