[PATCH] powerpc/vas: do not set uses_vas for kernel windows

[PATCH] powerpc/vas: do not set uses_vas for kernel windows

[Nicholas Piggin]

cp_abort is only required or user windows, because kernel context
must not be preempted between a copy/paste pair.

Without this patch, the init task gets used_vas set when it runs
the nx842_powernv_init initcall, which opens windows for kernel
usage.

used_vas is then never cleared anywhere, so it gets propagated
into all other tasks. It's a property of the address space, so it
should really be cleared when a new mm is created (or in dup_mmap
if the mmaps are marked as VM_DONTCOPY). For now we seem to have
no such driver, so leave that for another patch.

Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/platforms/powernv/vas-window.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/vas-window.c b/arch/powerpc/platforms/powernv/vas-window.c
index 2b3eb01ab110..b7c53a51c31b 100644
--- a/arch/powerpc/platforms/powernv/vas-window.c
+++ b/arch/powerpc/platforms/powernv/vas-window.c
@@ -1063,16 +1063,16 @@ struct vas_window *vas_tx_win_open(int vasid, enum vas_cop_type cop,
 			rc = PTR_ERR(txwin->paste_kaddr);
 			goto free_window;
 		}
+	} else {
+		/*
+		 * A user mapping must ensure that context switch issues
+		 * CP_ABORT for this thread.
+		 */
+		rc = set_thread_uses_vas();
+		if (rc)
+			goto free_window;
 	}
 
-	/*
-	 * Now that we have a send window, ensure context switch issues
-	 * CP_ABORT for this thread.
-	 */
-	rc = -EINVAL;
-	if (set_thread_uses_vas() < 0)
-		goto free_window;
-
 	set_vinst_win(vinst, txwin);
 
 	return txwin;
-- 
2.15.1

Re: [PATCH] powerpc/vas: do not set uses_vas for kernel windows

[Sukadev Bhattiprolu]

Nicholas Piggin [npiggin@gmail.com] wrote:
> cp_abort is only required or user windows, because kernel context
> must not be preempted between a copy/paste pair.

Yes, that is a good optimization.

> 
> Without this patch, the init task gets used_vas set when it runs
> the nx842_powernv_init initcall, which opens windows for kernel
> usage.
> 
> used_vas is then never cleared anywhere, so it gets propagated
> into all other tasks. It's a property of the address space, so it
> should really be cleared when a new mm is created (or in dup_mmap
> if the mmaps are marked as VM_DONTCOPY). For now we seem to have
> no such driver, so leave that for another patch.

If the parent process has the paste address mapped, the child inherits
those mappings - so we can't clear the ->used_vas in a process until
it has unmapped all the send windows right?

If VM_DONCOPY is set, then we can clear it.

> 
> Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Reviewed-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>

Re: powerpc/vas: do not set uses_vas for kernel windows

[Michael Ellerman]

On Thu, 2018-02-08 at 09:18:38 UTC, Nicholas Piggin wrote:
> cp_abort is only required or user windows, because kernel context
> must not be preempted between a copy/paste pair.
> 
> Without this patch, the init task gets used_vas set when it runs
> the nx842_powernv_init initcall, which opens windows for kernel
> usage.
> 
> used_vas is then never cleared anywhere, so it gets propagated
> into all other tasks. It's a property of the address space, so it
> should really be cleared when a new mm is created (or in dup_mmap
> if the mmaps are marked as VM_DONTCOPY). For now we seem to have
> no such driver, so leave that for another patch.
> 
> Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> Reviewed-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/b00b62898631b756c3e123542bbb04

cheers