Re: [RFC 01/20] mm/tlb: fix fullmm semantics

Nadav Amit
2021-02-02T21:37:01Z

> On Feb 2, 2021, at 3:00 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Feb 02, 2021 at 01:32:36AM -0800, Nadav Amit wrote:
>>> On Feb 1, 2021, at 3:36 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>
>>>
>>> https://lkml.kernel.org/r/20210127235347.1402-1-will@kernel.org
>>
>> I have seen this series, and applied my patches on it.
>>
>> Despite Will’s patches, there were still inconsistencies between fullmm
>> and need_flush_all.
>>
>> Am I missing something?
>
> I wasn't aware you were on top. I'll look again.

Looking on arm64’s tlb_flush() makes me think that there is currently a bug
that this patch fixes. Arm64’s tlb_flush() does:

/*
* If we're tearing down the address space then we only care about
* invalidating the walk-cache, since the ASID allocator won't
* reallocate our ASID without invalidating the entire TLB.
*/
if (tlb->fullmm) {
if (!last_level)
flush_tlb_mm(tlb->mm);
return;
}

But currently tlb_mmu_finish() can mistakenly set fullmm incorrectly (if
mm_tlb_flush_nested() is true), which might skip the TLB flush.

Lucky for us, arm64 flushes each VMA separately (which as we discussed
separately may not be necessary), so the only PTEs that might not be flushed
are PTEs that are updated concurrently by another thread that also defer
their flushes. It therefore seems that the implications are more on the
correctness of certain syscalls (e.g., madvise(DONT_NEED)) without
implications on security or memory corruptions.

Let me know if you want me to send this patch separately with an updated
commit log for faster inclusion.