Re: [PATCH] powerpc/mm: Fix set_memory_*() against concurrent accesses

Michael Ellerman [off-list ref] writes:

Hi, I already mentioned these things in private, but I'll post here so
everyone can see:

Because pte_update() takes the set of PTE bits to set and clear we can't
use our existing helpers, eg. pte_wrprotect() etc. and instead have to
open code the set of flags. We will clean that up somehow in a future
commit.

I tested the following on P9 and it seems to work fine. Not sure if it
works for CONFIG_PPC_8xx, though.


 static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
 {
 	long action = (long)data;
 	pte_t pte;
 
 	spin_lock(&init_mm.page_table_lock);
-
-	/* invalidate the PTE so it's safe to modify */
-	pte = ptep_get_and_clear(&init_mm, addr, ptep);
-	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+	pte = *ptep;
 
 	/* modify the PTE bits as desired, then apply */
 	switch (action) {

@@ -59,11 +42,9 @@ static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
 		break;
 	}
 
-	set_pte_at(&init_mm, addr, ptep, pte);
+	pte_update(&init_mm, addr, ptep, ~0UL, pte_val(pte), 0);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
 
-	/* See ptesync comment in radix__set_pte_at() */
-	if (radix_enabled())
-		asm volatile("ptesync": : :"memory");
 	spin_unlock(&init_mm.page_table_lock);
 
 	return 0;
---

For reference, the full patch is here:
https://github.com/farosas/linux/commit/923c95c84d7081d7be9503bf5b276dd93bd17036.patch

[1]: https://lore.kernel.org/linuxppc-dev/87y318wp9r.fsf@linux.ibm.com/ (local)

Fixes: 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines")
Reported-by: Laurent Vivier <redacted>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---

...

-	set_pte_at(&init_mm, addr, ptep, pte);
+	pte_update(&init_mm, addr, ptep, clear, set, 0);
 
 	/* See ptesync comment in radix__set_pte_at() */
 	if (radix_enabled())
 		asm volatile("ptesync": : :"memory");
+
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);

I think there's an optimization possible here, when relaxing access, to
skip the TLB flush. Would still need the ptesync though. Similar to what
Nick did in e5f7cb58c2b7 ("powerpc/64s/radix: do not flush TLB when
relaxing access"). It is out of scope for this patch but maybe worth
thinking about.

+
 	spin_unlock(&init_mm.page_table_lock);
 
 	return 0;

base-commit: cbc06f051c524dcfe52ef0d1f30647828e226d30