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

Nicholas Piggin
2019-06-04T08:18:46Z

Will Deacon's on June 4, 2019 3:57 am:
> Hi Nick,
>
> On Tue, Jun 04, 2019 at 12:10:37AM +1000, Nicholas Piggin wrote:
>> Will Deacon's on June 3, 2019 8:30 pm:
>> > On Mon, Jun 03, 2019 at 12:11:38PM +1000, Nicholas Piggin wrote:
>> >> Peter Zijlstra's on May 31, 2019 7:49 pm:
>> >> > On Fri, May 31, 2019 at 12:46:56PM +1000, Nicholas Piggin wrote:
>> >> >> 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.
>> >
>> > I honestly don't think we (Peter and I) know. Our first involvement with
>> > this was because TLBs were found to contain stale user entries:
>> >
>> > https://lore.kernel.org/linux-arm-kernel/1817839533.20996552.1557065445233.JavaMail.zimbra@redhat.com/
>> >
>> > so the initial work to support the concurrent freeing was done separately
>> > and, I assume, motivated by some real workloads. I would also very much
>> > like to know more about that, since nothing remotely realistic has surfaced
>> > in this discussion, other than some historical glibc thing which has long
>> > since been fixed.
>>
>> Well, it seems like it is important. While the complexity is carried
>> in the mm, we should not skimp on this last small piece.
>
> As I say, I really don't know. But yes, if we can do something better we
> should.
>
>> >> >> 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.
>> >
>> > I think you're correct that two CPUs cannot free the page tables
>> > concurrently (I misunderstood this initially), although I also think
>> > there may be some subtle issues if tlb->freed_tables is not set,
>> > depending on the architecture. Roughly speaking, if one CPU is clearing
>> > a PMD as part of munmap() and another CPU in madvise() does only last-level
>> > TLB invalidation, then I think there's the potential for the invalidation
>> > to be ineffective if observing a cleared PMD doesn't imply that the last
>> > level has been unmapped from the perspective of the page-table walker.
>>
>> That should not be the case because the last level table should have
>> had all entries cleared before the pointer to it has been cleared.
>
> The devil is in the detail here, and I think specifically it depends
> what you mean by "before". Does that mean memory barrier, or special
> page-table walker barrier, or TLB invalidation or ...?

I don't know that matters. It is a complicating factor, but would
not be a new one to page table freeing. The issue really is the
TLB entries (not page walk entry) which need to be flushed by the
concurrent unmaps. Even without any page table freeing activity (so you
would still have the page table connected), the ordering and flushing
needs to be right such that it can not re-instantiate a new TLB from the
page table with the old PTEs.

If that is solved, then the subsequent step of freeing the page table
page doesn't introduce a new window where old PTEs could be read
from the same table via page walk cache after it is disconnected.

>
>> So the page table walker could begin from the now-freed page table,
>> but it would never instantiate a valid TLB entry from there. So a
>> TLB invalidation would behave properly despite not flushing page
>> tables.
>>
>> Powerpc at least would want to avoid over flushing here, AFAIKS.
>
> For arm64 it really depends how often this hits. Simply not setting
> tlb->freed_tables would also break things for us, because we have an
> optimisation where we elide invalidation in the fullmm && !freed_tables
> case, since this is indicative of the mm going away and so we simply
> avoid reallocating its ASID.

It wouldn't be not setting it, but rather not clearing it.

Not sure you have to worry about concurrent unmaps in the fullmm case
either.

>
>> >> > 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.
>> >
>> > If we can get a better idea of what a "good benchmark" might look like (i.e.
>> > something that is representative of the cases in which real workloads are
>> > likely to trigger this path) then we can definitely try to optimise around
>> > that.
>>
>> Hard to say unfortunately. A smaller unmap range to start with, but
>> even then when you have a TLB over-flushing case, then an unmap micro
>> benchmark is not a great test because you'd like to see more impact of
>> other useful entries being flushed (e.g., you need an actual working
>> set).
>
> Right, sounds like somebody needs to do some better analysis than what's
> been done so far.
>
>> > In the meantime, I would really like to see this patch land in mainline
>> > since it fixes a regression.
>>
>> Sorry I didn't provide input earlier. I would like to improve the fix or
>> at least make an option for archs to provide an optimised way to flush
>> this case, so it would be nice not to fix archs this way and then have
>> to change the fix significantly right away.
>
> Please send patches ;)

I have a few things lying around I put on hold until after all the
recent nice refactoring and improvements. I will rebase and try to
look at this issue as well and see if there is any ways to improve.
Likely to need help with arch code and analysis of races.

>> But the bug does need to be fixed of course, if there needs to be more
>> thought about it maybe it's best to take this fix for next release.
>
> Agreed.

Well I can't argue against in anymore then.

Thanks,
Nick