[PATCH v33 05/14] arm64: mm: allow for unmapping part of kernel mapping

On Tue, Mar 21, 2017 at 10:35:53AM +0000, James Morse wrote:

Hi Akashi,

On 15/03/17 09:59, AKASHI Takahiro wrote:

quoted

create_pgd_mapping() is enhanced here so that it will accept
PAGE_KERNEL_INVALID protection attribute and unmap a given range of memory.

The feature will be used in a later kdump patch to implement the protection
against possible corruption of crash dump kernel memory which is to be set
aside from ther other memory on primary kernel.

Nit: ther- > the

Fix it.

quoted

Note that, in this implementation, it assumes that all the range of memory
to be processed is mapped in page-level since the only current user is
kdump where page mappings are also required.

Using create_pgd_mapping() like this means the mappings will be updated via the
fixmap which is unnecessary as the page tables will be part of mapped memory. In

This might be a reason that we would go for (un)map_kernel_range()
over create_pgd_mapping() (? not sure)

the worst case this adds an extra tlbi for every 2MB of crash image when we map
or unmap it. I don't think this matters.

This code used to be __init and it is the only user of FIX_PTE, so there won't
be any existing runtime users. The two arch_kexec_unprotect_crashkres() calls in
kexec are both protected by the kexec_mutex, and the call in hibernate happens
after disable_nonboot_cpus(), so these callers can't race with each other.

This looks safe to me.

quoted

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index d28dbcf596b6..cb359a3927ef 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -128,7 +128,10 @@ static void alloc_init_pte(pmd_t *pmd, unsigned long addr,
 	do {
 		pte_t old_pte = *pte;
 
-		set_pte(pte, pfn_pte(pfn, prot));
+		if (pgprot_val(prot))
+			set_pte(pte, pfn_pte(pfn, prot));
+		else
+			pte_clear(null, null, pte);

Lowercase NULLs? This relies on these values never being used... __set_fixmap()
in the same file passes &init_mm and the address, can we do the same to be
consistent?

OK.

Thanks,
-Takahiro AKASHI

quoted

 		pfn++;
 
 		/*

Reviewed-by: James Morse <james.morse@arm.com>


Thanks,

James