Re: [PATCH 1/2] arm64: Get existing page protections in split_pmd

From: Laura Abbott
Date: Thu Nov 05 2015 - 20:24:11 EST


On 11/05/2015 02:15 AM, Xishi Qiu wrote:
On 2015/11/4 5:48, Laura Abbott wrote:


Rather than always putting the least restrictived permissions
(PAGE_KERNEL_EXEC) when spliting a pmd into pages, use
the existing permissions from the pmd for the page.

Signed-off-by: Laura Abbott <labbott@xxxxxxxxxxxxxxxxx>
---
arch/arm64/mm/mmu.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9211b85..ff41efa 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -75,14 +75,13 @@ static void __init *early_alloc(unsigned long sz)
static void split_pmd(pmd_t *pmd, pte_t *pte)
{
unsigned long pfn = pmd_pfn(*pmd);
+ unsigned long addr = pfn << PAGE_SHIFT;
+ pgprot_t prot = __pgprot(pmd_val(*pmd) ^ addr) | PTE_TYPE_PAGE;
+

Hi Laura,

I'm not quite understand, I find split_pud() doesn't set the flag
PMD_TYPE_TABLE. If we clear xx_TABLE_BIT, does that means the page
is large page?

I'm assuming by large page you mean a block mapping. Yes, without
that entry the kernel treats this as a block mapping

And what is the different from the flag xx_TYPE_SECT?

That would mark it this as a block mapping which is not what we want
here.


Thanks,
Xishi Qiu




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