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

[Shile Zhang]

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