Re: [uml-devel] Re: [uml-user] /dev/anon available

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

Alle 05:54, domenica 18 gennaio 2004, Jeff Dike ha scritto:
> On Sat, Jan 17, 2004 at 08:48:47PM +0100, BlaisorBlade wrote:
> > I've given a look to the page, but yet I cannot see (this can be for my
> > fault, but I'd really like an explaination of this) why you couldn't just
> > mmap /dev/zero with MAP_PRIVATE to get the behaviour you want to get from
> > /dev/anon (or maybe using MAP_ANONYMOUS).
>
> Because when you map a /dev/zero page, modify it, then map it (i.e. the
> same offset and the same fd) again someplace else, you don't get the
> modifications. You get a page full of zeros.   This obviously breaks the
> UML VM system.

Yes, that's obvious with MAP_PRIVATE (and using MAP_SHARED would be 
meaningless, if at all worked). But I wasn't aware you needed to map the same 
region twice; I guess this is done for pages shared (even with copy on write) 
between different processes inside UML, right? Also, since with skas the 
mm_struct's of the virtual processes have nothing in common, they cannot

However, have you asked to Linus if he likes /dev/anon? Maybe he'll suggest 
another interface for the same need (like when you two discussed skas4).

-- 
cat <<EOSIGN
Paolo Giarrusso, aka Blaisorblade
Linux Kernel 2.4.23/2.6.0 on an i686; Linux registered user n. 292729
EOSIGN




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