arm linux page initialization

[Russell King - ARM Linux]

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On Thu, Oct 13, 2016 at 01:09:08PM +0200, Oscar Salvador wrote:
> Hi all, I have a question regarding page initialization on arm32 without
> LPAE support enabled.
> 
> I was trying to find the right pte of a kernel linear address (f.i
> 0xc05518bc)
> by walking the page tables starting with the pgd taken from TTBR1.
> 
> I was using the functions:
> pgd_offset_k
> pud_offset
> pmd_offset
> pte_offset_kernel
> 
> Since pte_offset_kernel should lead me to virtual address of the Linux
> PTE version, I thougt I should be able to check the L_PTE_* bits.
> But actually I realized that the pte given from pte_offset_kernel
> didn't have such flags.

You need to check for "bad" pgd/pud/pmd entries as you walk down, which
will exclude things like sections.

> By looking at the code from arch/arm/mm/mmu.c, I could see that, actually
> (talking about a system with 1GB ram), during map_lowmem function,
> memory (virtual) from 0xC0000000 to 0xef800000 is being put in sections
> of 1MB and marked as a "PMD_TYPE_SECT | PMD_SECT_AP_WRITE",
> then it does a flush_pmd_entry.
> 
> So that means that the first level table for these addresses are pointing
> directly to a Section, instead to point to a second level tables, right?

Correct.  We map the kernel using sections not page tables, because it
is much more efficient.

> If that's true, I guess this memory don't have PTE's
> (at least I didn't see that at boot time).
> Given that, without pte's available for this range of memory,
> we shoudln't be able to check for L_PTE_* flags here.
> 
> Then, why do we have all these L_PTE_* flags?
> Are not they being used to check for instance if the kernel code is
> writeable or not (I think you can configure that).

Just because the kernel is mapped with sections doesn't mean that we only
use sections everywhere.  We use page tables for user mappings.

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