Re: [PATCH] mm/vmalloc: Fix regression caused by needless vmalloc_sync_all()

From: Shile Zhang
Date: Thu Nov 14 2019 - 09:40:35 EST




On 2019/11/14 21:56, Michal Hocko wrote:
On Wed 13-11-19 17:55:30, Shile Zhang wrote:
vmalloc_sync_all() was put in the common path in
__purge_vmap_area_lazy(), for one sync issue only happened on X86_32
with PTI enabled. It is needless for X86_64, which caused a big regression
in UnixBench Shell8 testing on X86_64.
Similar regression also reported by 0-day kernel test robot in reaim
benchmarking:
https://lists.01.org/hyperkitty/list/lkp@xxxxxxxxxxxx/thread/4D3JPPHBNOSPFK2KEPC6KGKS6J25AIDB/

Fix it by adding more conditions.
This doesn't really explain a lot. Could you explain why the syncing
should be limited only to PAE and !SHARED_KERNEL_PMD?

Thanks for your review!
Sorry for that, I'm not clear about the original issue the patch
3f8fd02b1bf1 ("mm/vmalloc: Sync unmappings in __purge_vmap_area_lazy()")
fixed.

I just get that limitation info from the commit log as following:
"
This is only needed x86-32 with !SHARED_KERNEL_PMD, which is the case on a PAE kernel with PTI enabled. On affected systems the missing sync causes old mappings to persist in some page-tables, causing data corruption and other undefined behavior.
"
https://patchwork.kernel.org/cover/11050511/

Hi, Joerg
Could you please help to recall the original issue you encountered before?
Thanks!


Fixes: 3f8fd02b1bf1 ("mm/vmalloc: Sync unmappings in __purge_vmap_area_lazy()")

Signed-off-by: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
---
mm/vmalloc.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a3c70e275f4e..7b9fc7966da6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1255,11 +1255,17 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
if (unlikely(valist == NULL))
return false;
+#if defined(CONFIG_X86_32) && defined(CONFIG_X86_PAE)
/*
* First make sure the mappings are removed from all page-tables
* before they are freed.
+ *
+ * This is only needed on x86-32 with !SHARED_KERNEL_PMD, which is
+ * the case on a PAE kernel with PTI enabled.
*/
- vmalloc_sync_all();
+ if (!SHARED_KERNEL_PMD && boot_cpu_has(X86_FEATURE_PTI))
+ vmalloc_sync_all();
+#endif
/*
* TODO: to calculate a flush range without looping.
--
2.24.0.rc2