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

[Nicholas Piggin <npiggin@gmail.com>]

Peter Zijlstra's on May 31, 2019 7:49 pm:
> 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.

I mean -- concurrent freeing was an important fastpath, right?
And concurrent freeing means that you hit this case. So this
case itself should be important too.

> 
>> 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..).

Page tables should not be concurrently freed I think. Just don't clear
those page table free flags and it should be okay. Page sizes yes,
but we accommodated for that in the arch code. I could see reason to
add a flag to the gather struct like "concurrent_free" and set that
from the generic code, which the arch has to take care of.

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

Testing on 16MB unmap is possibly not a good benchmark, I didn't run
it exactly but it looks likely to go beyond the range flush threshold
and flush the entire PID anyway.

Thanks,
Nick