Re: [PATCH] powerpc/mm: bail out early when flushing TLB page
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: 2015-01-31 20:30:13
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On Fri, 2015-01-30 at 19:08 +0700, Arseny Solokha wrote:
MMU_NO_CONTEXT is conditionally defined as 0 or (unsigned int)-1. However, in __flush_tlb_page() a corresponding variable is only tested for open coded 0, which can cause NULL pointer dereference if `mm' argument was legitimately passed as such. Bail out early in case the first argument is NULL, thus eliminate confusion between different values of MMU_NO_CONTEXT and avoid disabling and then re-enabling preemption unnecessarily.
So the comment above isn't quite right... we don't *test* it for open coded 0, we test it for MMU_NO_CONTEXT, however we *set* it to 0 for NULL mm. This is actually correct... on all except 8xx :-) 0 *is* the PID of the kernel context, and NULL mm usually means kernel context. However, it's correct that this function will not deal properly with a NULL mm for other reasons. It must only be called for user contexts. Instead of just returning, I would WARN_ON, because if it's ever called for a kernel page, then it will not do what's expected and that will need fixing. Just a silent return isn't right. This is different from returning on MMU_NO_CONTEXT, in this case, we know there's no active TLB entries for the process, and thus nothing to flush. Cheers, Ben.
quoted hunk ↗ jump to hunk
Signed-off-by: Arseny Solokha <redacted> --- arch/powerpc/mm/tlb_nohash.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)diff --git a/arch/powerpc/mm/tlb_nohash.c b/arch/powerpc/mm/tlb_nohash.c index f38ea4d..ab0616b 100644 --- a/arch/powerpc/mm/tlb_nohash.c +++ b/arch/powerpc/mm/tlb_nohash.c@@ -284,8 +284,11 @@ void __flush_tlb_page(struct mm_struct *mm, unsigned long vmaddr, struct cpumask *cpu_mask; unsigned int pid; + if (unlikely(!mm)) + return; + preempt_disable(); - pid = mm ? mm->context.id : 0
Here we test mm, if we pass NULL, that means the kernel mm which has PID 0, which is not MMU_NO_CONTEXT
; + pid = mm->context.id; if (unlikely(pid == MMU_NO_CONTEXT)) goto bail; cpu_mask = mm_cpumask(mm);