[Question] A question about arm64 pte

[catalin.marinas@arm.com (Catalin Marinas)]

On Tue, Jan 17, 2017 at 11:53:43AM +0800, Yisheng Xie wrote:
> On 2017/1/16 22:36, Catalin Marinas wrote:
> > On Mon, Jan 16, 2017 at 08:39:56PM +0800, Yisheng Xie wrote:
> >> On 2017/1/16 19:56, Catalin Marinas wrote:
> >>> On Mon, Jan 16, 2017 at 06:08:47PM +0800, Yisheng Xie wrote:
> >>>
> >> However?when use memset to write the region it still works well, and
> >> the bit PTE_RDONLY is also cleared. Is there anywhere clear the
> >> PTE_RDONLY before write that page ?
> > 
> > See handle_pte_fault(). On the first access to a writable+clean page
> > (PTE_WRITE set, PTE_RDONLY set, PTE_DIRTY cleared), the kernel traps it
> > and, if pte_write() is true (your case), it calls pte_mkdirty(). The
> > subsequently called ptep_set_access_flags() function would clear
> > PTE_RDONLY, giving you a writable mapping.
> 
> Sorry to disturb, but why page fault will happened here, for pte already
> present with AF bit set?
> 
> Here is what I get when mmap a reserved memory region 0x39ef 0000~0x3a00 0000
> use /dev/mem:
> [  442.704228] pgd = ffff802785f14000
> [  442.707641] [ffff86e4b000] *pgd=000000279080c003, *pud=0000002785f01003, *pmd=0000002783f5b003, *pte=0168000039ef0fd3

The pte seems to have bit 7 set: PTE_RDONLY. So you would get a fault on
write (but not read). Since PTE_WRITE is also set, handle_pte_fault()
will mark the entry as dirty (bit 55, cleared above) and clear
PTE_RDONLY before restarting the access instruction. User space wouldn't
notice, just a slight delay on the first access.

-- 
Catalin