translation table APTable permission problem

Hello,

I am trying to make pmd level of the translation table as read only so that any writes in the pte entries should cause a permission fault.

Current steps which i am doing are:

1) inside kernel space, allocating 2 pointers (say p, q) and allocating memory using vmalloc (4k size to occupy entire page)

2) assigning int data to both (say p=10, q= 20)

3) setting APTable bit for readonly for both the pointers (By reading manual, bit 61 and 62 denotes APTable)

4) swapping pte value for both the pointer in translation table

5) printing value for both the pointer

At step 4, access permission fault is not occurring.

I tried flushing the TLB, still at point 4 fault does not occur, but fault is occurring at random places (sometimes after point 5 or in between any points)

Can anyone tell me possible reasons why this is happening or some solution to fix it?

thanks