[PATCH 4.4 37/75] arm64: mm: always take dirty state from new pte in ptep_set_access_flags

Greg Kroah-Hartman
2016-06-22T23:15:56Z

From: Will Deacon <will.deacon@arm.com>

commit 0106d456c4cb1770253fefc0ab23c9ca760b43f7 upstream.

Commit 66dbd6e61a52 ("arm64: Implement ptep_set_access_flags() for
hardware AF/DBM") ensured that pte flags are updated atomically in the
face of potential concurrent, hardware-assisted updates. However, Alex
reports that:

| This patch breaks swapping for me.
| In the broken case, you'll see either systemd cpu time spike (because
| it's stuck in a page fault loop) or the system hang (because the
| application owning the screen is stuck in a page fault loop).

It turns out that this is because the 'dirty' argument to
ptep_set_access_flags is always 0 for read faults, and so we can't use
it to set PTE_RDONLY. The failing sequence is:

1. We put down a PTE_WRITE | PTE_DIRTY | PTE_AF pte
2. Memory pressure -> pte_mkold(pte) -> clear PTE_AF
3. A read faults due to the missing access flag
4. ptep_set_access_flags is called with dirty = 0, due to the read fault
5. pte is then made PTE_WRITE | PTE_DIRTY | PTE_AF | PTE_RDONLY (!)
6. A write faults, but pte_write is true so we get stuck

The solution is to check the new page table entry (as would be done by
the generic, non-atomic definition of ptep_set_access_flags that just
calls set_pte_at) to establish the dirty state.

Fixes: 66dbd6e61a52 ("arm64: Implement ptep_set_access_flags() for hardware AF/DBM")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Alexander Graf <agraf@suse.de>
Tested-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm64/mm/fault.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -109,7 +109,7 @@ int ptep_set_access_flags(struct vm_area
* PTE_RDONLY is cleared by default in the asm below, so set it in
* back if necessary (read-only or clean PTE).
*/
- if (!pte_write(entry) || !dirty)
+ if (!pte_write(entry) || !pte_sw_dirty(entry))
pte_val(entry) |= PTE_RDONLY;

/*