Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

Peter Zijlstra
2019-05-09T10:54:58Z

On Thu, May 09, 2019 at 12:38:13PM +0200, Peter Zijlstra wrote:

> That's tlb->cleared_p*, and yes agreed. That is, right until some
> architecture has level dependent TLBI instructions, at which point we'll
> need to have them all set instead of cleared.

> Anyway; am I correct in understanding that the actual problem is that
> we've cleared freed_tables and the ARM64 tlb_flush() will then not
> invalidate the cache and badness happens?
>
> Because so far nobody has actually provided a coherent description of
> the actual problem we're trying to solve. But I'm thinking something
> like the below ought to do.

There's another 'fun' issue I think. For architectures like ARM that
have range invalidation and care about VM_EXEC for I$ invalidation, the
below doesn't quite work right either.

I suspect we also have to force: tlb->vma_exec = 1.

And I don't think there's an architecture that cares, but depending on
details I can construct cases where any setting of tlb->vm_hugetlb is
wrong, so that is _awesome_. But I suspect the sane thing for now is to
force it 0.

> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index 99740e1dd273..fe768f8d612e 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -244,15 +244,20 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
> unsigned long start, unsigned long end)
> {
> /*
> - * If there are parallel threads are doing PTE changes on same range
> - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
> - * flush by batching, a thread has stable TLB entry can fail to flush
> - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
> - * forcefully if we detect parallel PTE batching threads.
> + * Sensible comment goes here..
> */
> - if (mm_tlb_flush_nested(tlb->mm)) {
> - __tlb_reset_range(tlb);
> - __tlb_adjust_range(tlb, start, end - start);
> + if (mm_tlb_flush_nested(tlb->mm) && !tlb->full_mm) {
> + /*
> + * Since we're can't tell what we actually should have
> + * flushed flush everything in the given range.
> + */
> + tlb->start = start;
> + tlb->end = end;
> + tlb->freed_tables = 1;
> + tlb->cleared_ptes = 1;
> + tlb->cleared_pmds = 1;
> + tlb->cleared_puds = 1;
> + tlb->cleared_p4ds = 1;
> }
>
> tlb_flush_mmu(tlb);