Patch "arm64: mm: always take dirty state from new pte in ptep_set_access_flags" has been added to the 4.6-stable tree

This is a note to let you know that I've just added the patch titled

arm64: mm: always take dirty state from new pte in ptep_set_access_flags

to the 4.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-mm-always-take-dirty-state-from-new-pte-in-ptep_set_access_flags.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Will Deacon <[email protected]>
Date: Tue, 7 Jun 2016 17:55:15 +0100
Subject: arm64: mm: always take dirty state from new pte in ptep_set_access_flags

From: Will Deacon <[email protected]>

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.

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

@@ -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;
 
 	/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.6/drivers-perf-arm_pmu-defer-the-setting-of-__oprofile_cpu_pmu.patch queue-4.6/arm64-provide-model-name-in-proc-cpuinfo-for-per_linux32-tasks.patch queue-4.6/arm64-mm-always-take-dirty-state-from-new-pte-in-ptep_set_access_flags.patch queue-4.6/arm-fix-ptrace_setvfpregs-on-smp-systems.patch