Re: Doubt regarding memory

[Tzahi Fadida <Tzahi.ML2@xxxxxxxxx>]

On Thursday 01 March 2007 07:10, Mulyadi Santosa wrote:
> Hello Tzahi
>
> You really give me tough Q :)

:), hopefully we are both learning :).

> 6. Before page are returned to user space, the related page tables that
> points to these pages are manipulated so that they are using user space
> virtual address space range (the clue is by looking where the fault did
> happen)
>
> Note those pages now could have two mapping (if they lies inside normal
> and DMA zone), one is via identity mapping, one is that process's own
> mapping

Does that mean that kernel memory is swappable/context switched. I.e. when the 
process is "current", he can address the same physical address the kernel 
will when the process is still in the run queue but not "current".

>
> > Let's say we have 2gb ram.
> > If the kernel already allocated it's 3-4gb (1gb) but it wants more, it
> > will have to resort to taking from virtual user space addresses, right?
> > but they won't be protected so what gives?
>
> If the kernel wants more, it can do so via temporary mapping (kmap()).
> By doing it, the highmem pages will be temporarily mapped in the area
> slightly beyond the end of kernel identity mapping (AFAIK, this is the
> one we know as the upper 128 MB area, use for vmalloc, fixed linear

I think vmalloc also takes first from the normal zone (since it actually uses 
kmalloc IIRC).
Also, i am trying to understand this identity mapping. I understand it's 
simplicity, but how the MMU uses this fact? is it still uses page tables for 
this? since this would be very wasteful. 
In addition, i understand that in (no pae) x86 32bit you have 1MB of 4 bytes 
entries that can be referenced by the MMU (which, i think, can only address 
1gb of ram pages). Does this mean that the kernel has to change the page 
tables as necessary, i.e. when a process does context switch and needs the 
same virtual page address used by a different process. Does the kernel leaves 
there (in the MMU current page tables) some entries of its own constantly?

> I wish my explanation really clears the confusion, because once I
> struggled a lot with these things too and slowly I got the idea.

Yes 10x, this things are complicated. 

-- 
Regards,
        Tzahi.
--
Tzahi Fadida
Blog: http://tzahi.blogsite.org | Home Site: http://tzahi.webhop.info
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