[PATCH] arm64: mm: Fix false positives in set_pte_at access/dirty race detection

[Will Deacon]

Hi Yisheng,

On Wed, Dec 13, 2017 at 09:01:23AM +0800, Yisheng Xie wrote:
> On 2017/12/12 19:43, Will Deacon wrote:
> > Jiankang reports that our race detection in set_pte_at is firing when
> > copying the page tables in dup_mmap as a result of a fork(). In this
> > situation, the page table isn't actually live and so there is no way
> > that we can race with a concurrent update from the hardware page table
> > walker.
> > 
> > This patch reworks the race detection so that we require either the
> > mm to match the current active_mm (i.e. currently installed in our TTBR0)
> > or the mm_users count to be greater than 1, implying that the page table
> > could be live in another CPU. The mm_users check might still be racy,
> > but we'll avoid false positives and it's not realistic to validate that
> > all the necessary locks are held as part of this assertion.
> > 
> > Cc: Yisheng Xie <xieyisheng1 at huawei.com>
> > Reported-by: Jiankang Chen <chenjiankang1 at huawei.com>
> > Tested-by: Jiankang Chen <chenjiankang1 at huawei.com>
> > Signed-off-by: Will Deacon <will.deacon at arm.com>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 3ff03a755c32..bdcc7f1c9d06 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -42,6 +42,8 @@
> >  #include <asm/cmpxchg.h>
> >  #include <asm/fixmap.h>
> >  #include <linux/mmdebug.h>
> > +#include <linux/mm_types.h>
> > +#include <linux/sched.h>
> >  
> 
> Do you have compiled kernel after apply this patch? In our environment, it will
> fail to compile kernel if include file here(I will attach some log later).
> Instead, we move these included file after mmdebug.h, and I do not know whether
> this is just my compiler's problem:

It compiles fine for me. Are you seeing a problem building this on top of
mainline? If so, what is your .config?

>   --- a/arch/arm64/include/asm/pgtable.h
>   +++ b/arch/arm64/include/asm/pgtable.h
>   @@ -42,6 +42,8 @@
>    #include <asm/cmpxchg.h>
>    #include <asm/fixmap.h>
>    #include <linux/mmdebug.h>
>    +#include <linux/mm_types.h>
>    +#include <linux/sched.h>
> 
> Sorry for not having told you this information.
> 
> Thanks
> Yisheng Xie
> 
> compiler err log: ==========
> [...]
> include/linux/mm_types_task.h:60: Error: unknown mnemonic `struct' -- `struct page_frag{'
> include/asm-generic/preempt.h:9: Error: unknown mnemonic `static' -- `static inline int preempt_count(void)'
> include/linux/mm_types_task.h:61: Error: unknown mnemonic `struct' -- `struct page*page'
> include/asm-generic/preempt.h:10: Error: junk at end of line, first unrecognized character is `{'
> include/linux/mm_types_task.h:63: Error: unknown mnemonic `__u32' -- `__u32 offset'
> include/asm-generic/preempt.h:11: Error: unknown mnemonic `return' -- `return READ_ONCE(((struct thread_info*)current)->preempt_count)'
> include/linux/mm_types_task.h:64: Error: unknown mnemonic `__u32' -- `__u32 size'
> [...]

This looks like the includes are being pulled into an assembly file, but
this part is guarded by #ifndef __ASSEMBLY__ so I can't see how that could
happen.

Will