Re: [PATCH 2/2] iommu/amd: Remove clear_flush_young notifier

[Jean-Philippe Brucker <jean-philippe.brucker@arm.com>]

On 01/02/2019 03:51, Peter Xu wrote:
> On Thu, Jan 31, 2019 at 12:25:40PM +0000, Jean-Philippe Brucker wrote:
>> On 31/01/2019 07:59, Peter Xu wrote:
>>> On Wed, Jan 30, 2019 at 12:27:40PM +0000, Jean-Philippe Brucker wrote:
>>>> Hi Peter,
>>>
>>> Hi, Jean,
>>>
>>>>
>>>> On 30/01/2019 05:57, Peter Xu wrote:
>>>>> AMD IOMMU driver is using the clear_flush_young() to do cache flushing
>>>>> but that's actually already covered by invalidate_range().  Remove the
>>>>> extra notifier and the chunks.
>>>>
>>>> Aren't uses of clear_flush_young() and invalidate_range() orthogonal? If
>>>> I understood correctly, when doing reclaim the kernel clears the young
>>>> bit from the PTE. This requires flushing secondary TLBs (ATCs), so that
>>>> new accesses from devices will go through the IOMMU, set the young bit
>>>> again and the kernel can accurately track page use. I didn't see
>>>> invalidate_range() being called by rmap or vmscan in this case, but
>>>> might just be missing it.
>>>>
>>>> Two MMU notifiers are used for the young bit, clear_flush_young() and
>>>> clear_young(). I believe the former should invalidate ATCs so that DMA
>>>> accesses participate in PTE aging. Otherwise the kernel can't know that
>>>> the device is still using entries marked 'old'. The latter,
>>>> clear_young() is exempted from invalidating the secondary TLBs by
>>>> mmu_notifier.h and the IOMMU driver doesn't need to implement it.
>>>
>>> Yes I agree most of your analysis, but IMHO the problem is that the
>>> AMD IOMMU is not really participating in the PTE aging after all.
>>> Please have a look at mn_clear_flush_young() below at [1] - it's
>>> always returning zero, no matter whether the page has been accessed by
>>> device or not.  A real user of it could be someone like KVM (please
>>> see kvm_mmu_notifier_clear_flush_young) where we'll try to lookup the
>>> shadow PTEs and do test-and-clear on that, then return the result to
>>> the core mm.  That's why I think currently the AMD driver was only
>>> trying to use that as a way to do an extra flush however IMHO it's
>>> redundant.
>>
>> Yes, in IOMMU drivers clear_flush_young() doesn't do the clear, only the
>> flush, since the level-1 page table is shared with the CPU. But removing
>> the flush gets you in the following situation:
>>
>>   (1) Devices wants to access $addr, sends ATS query, the IOMMU sets PTE
>> young and the entry is cached in the ATC.
>>
>>   (2) The CPU does ptep_clear_flush_young_notify(), clears young,
>> notices that the page is being used.
>>
>>   (3) Device accesses $addr again. Given that we didn't invalidate the
>> ATC in (2) it accesses the page directly, without going through the IOMMU.
>>
>>   (4) CPU does ptep_clear_flush_young_notify() again, the PTE doesn't
>> have the young bit, which means the page isn't being used and can be
>> reclaimed.
>>
>> That's not accurate since the page is being used by the device. At step
>> (2) we should invalidate the ATC, so that (3) fetches the PTE again and
>> marks it young.
>>
>> I can agree that the clear_flush_young() notifier is too brutal for this
>> purpose, since we send spurious ATC invalidation even when the PTE
>> wasn't young (and ATC inv is expensive). There should be another MMU
>> notifier "flush_young()" that is called by
>> ptep_clear_flush_young_notify() only when the page was actually young.
>> But for the moment it's the best we have to avoid the situation above.
>>
>> I don't know enough about mm to understand exactly how the
>> page_referenced() information is used, but I believe the problem is only
>> about accuracy and not correctness. So applying this patch might not
>> break anything (after all, intel-svm.c never implemented the notifier)
>> but I think I'll keep the notifier in my SVA rework [1].
> 
> I see your point.  I'm not an expert of mm either, but I'd say AFAIU
> this happens in CPU TLB too.  Please have a look at
> ptep_clear_flush_young():
> 
> int ptep_clear_flush_young(struct vm_area_struct *vma,
> 			   unsigned long address, pte_t *ptep)
> {
> 	/*
> 	 * On x86 CPUs, clearing the accessed bit without a TLB flush
> 	 * doesn't cause data corruption. [ It could cause incorrect
> 	 * page aging and the (mistaken) reclaim of hot pages, but the
> 	 * chance of that should be relatively low. ]
> 	 *
> 	 * So as a performance optimization don't flush the TLB when
> 	 * clearing the accessed bit, it will eventually be flushed by
> 	 * a context switch or a VM operation anyway. [ In the rare
> 	 * event of it not getting flushed for a long time the delay
> 	 * shouldn't really matter because there's no real memory
> 	 * pressure for swapout to react to. ]
> 	 */
> 	return ptep_test_and_clear_young(vma, address, ptep);
> }

Aha I see. The arm64 version of ptep_clear_flush_young() does invalidate
the TLB if the PTE was young (perhaps because we don't invalidate the
TLB on context switch). For SVA I would have liked to simply invalidate
the ATC whenever the CPU invalidates its TLB, but that falls apart if
archs are doing different things...

> So maybe it is a tradeoff between performance and accuracy.  IMHO the
> IOMMU cache flushing might affect the performance even more than CPU
> TLB flushing if the invalidation command takes a long time to run
> (e.g., amd_iommu_flush_page is far slower than a TLB flush
> instruction, locks to take, queue commands, explicit wait for the
> invalidation to happen, etc.) so it can potentially even drag down the
> mm young bit access as a whole not to mention the future cache misses
> from the device side.
> 
> Even if you really want to make the young bit accurate for the SVA
> work, IMHO you may still want to implement the lightweight version of
> clear_young() too otherwise it might be inaccurate again in idle page
> tracking.

Yes there is another conversation about this on the new idle_pages
proposal [1], which would never send any ATC invalidation. I'm not sure
what we should do about it - making clear_young() flush the IOTLB seems
way too expensive there as well, we'll probably want something more
selective.

> Another thing to mention is that disregarding the discussion about
> young bit - IMO you should probably don't need the change_pte() which
> I'm more confident with.

I haven't dug too much into change_pte() yet, but I'll keep that in
mind, thanks!

Jean

[1] https://patchwork.kernel.org/cover/10743133/#22446425