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

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

Alle 06:10, martedì 13 gennaio 2004, Jeff Dike ha scritto:
> I released two /dev/anon patches today - one for stock 2.4.23 and one for
> RH8 2.4.8-14.
>
> In short, /dev/anon is a device which UML can map its physical memory from
> and which can free dirty pages back to the host.  In conjunction with
> 'ubd=mmap', this can noticably reduce the memory consumption of UMLs on
> the host.  See http://user-mode-linux.sourceforge.net/devanon.html for more
> details.
>
> This requires the next version of UML, which will be released shortly.

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). When you munmap a page, that goes 
out of your address space, so it cannot be dirty any more dirty. Also, with 
mmap you cannot modify /dev/zero, as it's a special case (mmap'ing that gives 
you a COW reference to the zero-filled page).

Beyond, I've checked that munmap works even for partial unmapping of files. So 
you could even mmap it as a single chunk (or better two chunks, read-only and 
read-write memory on i386; it would be harder on other arch's), at the end 
and munmap to free any page (that splits the old mapping into two new ones 
that map the pages around the unmapped ones). The problem is that using this 
means you cannot do mremap to enlarge mapped space, since in that case you 
don't know which mapping to remap (if you don't track it by hand, which would 
be actually hard).

Bye
-- 
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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