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 WARNING TO SPAMMERS: see at http://members.lycos.co.uk/my2nis/spamwarning.html -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
