Re: + mm-mmu_gather-remove-__tlb_reset_range-for-force-flush.patch added to -mm tree

Peter Zijlstra
2019-05-31T09:49:49Z

On Fri, May 31, 2019 at 12:46:56PM +1000, Nicholas Piggin wrote:
> Peter Zijlstra's on May 28, 2019 12:25 am:
> > On Mon, May 27, 2019 at 06:59:08PM +0530, Aneesh Kumar K.V wrote:
> >> On 5/27/19 4:31 PM, Peter Zijlstra wrote:
> >> > On Tue, May 21, 2019 at 04:18:33PM -0700, akpm@linux-foundation.org wrote:
> >> > > --- a/mm/mmu_gather.c~mm-mmu_gather-remove-__tlb_reset_range-for-force-flush
> >> > > +++ a/mm/mmu_gather.c
> >> > > @@ -245,14 +245,28 @@ void tlb_finish_mmu(struct mmu_gather *t
> >> > > {
> >> > > /*
> >> > > * 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.
> >> > > + * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB
> >> > > + * flush by batching, one thread may end up seeing inconsistent PTEs
> >> > > + * and result in having stale TLB entries. So flush TLB forcefully
> >> > > + * if we detect parallel PTE batching threads.
> >> > > + *
> >> > > + * However, some syscalls, e.g. munmap(), may free page tables, this
> >> > > + * needs force flush everything in the given range. Otherwise this
> >> > > + * may result in having stale TLB entries for some architectures,
> >> > > + * e.g. aarch64, that could specify flush what level TLB.
> >> > > */
> >> > > if (mm_tlb_flush_nested(tlb->mm)) {
> >> > > + /*
> >> > > + * The aarch64 yields better performance with fullmm by
> >> > > + * avoiding multiple CPUs spamming TLBI messages at the
> >> > > + * same time.
> >> > > + *
> >> > > + * On x86 non-fullmm doesn't yield significant difference
> >> > > + * against fullmm.
> >> > > + */
> >> > > + tlb->fullmm = 1;
> >> > > __tlb_reset_range(tlb);
> >> > > - __tlb_adjust_range(tlb, start, end - start);
> >> > > + tlb->freed_tables = 1;
> >> > > }
> >> > > tlb_flush_mmu(tlb);

> > Maybe, but given the patch that went into -mm, PPC will never hit that
> > branch I killed anymore -- and that really shouldn't be in architecture
> > code anyway.
>
> Yeah well if mm/ does this then sure it's dead and can go.
>
> I don't think it's very nice to set fullmm and freed_tables for this
> case though. Is this concurrent zapping an important fast path? It
> must have been, in order to justify all this complexity to the mm, so
> we don't want to tie this boat anchor to it AFAIKS?

I'm not convinced its an important fast path, afaict it is an
unfortunate correctness issue caused by allowing concurrenct frees.

> Is the problem just that the freed page tables flags get cleared by
> __tlb_reset_range()? Why not just remove that then, so the bits are
> set properly for the munmap?

That's insufficient; as argued in my initial suggestion:

https://lkml.kernel.org/r/20190509103813.GP2589@hirez.programming.kicks-ass.net

Since we don't know what was flushed by the concorrent flushes, we must
flush all state (page sizes, tables etc..).

But it looks like benchmarks (for the one test-case we have) seem to
favour flushing the world over flushing a smaller range.