Messages in this thread | | | Date | Sat, 2 Feb 2019 15:08:08 +0800 | From | Peter Xu <> | Subject | Re: [PATCH 2/2] iommu/amd: Remove clear_flush_young notifier |
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On Fri, Feb 01, 2019 at 11:46:00AM +0000, Jean-Philippe Brucker wrote: > 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!
Glad to have the discussion with you! Note that besides the one that was attached along with the same series, there's another patch to remove the last (besides KVM) change_pte() usage here in Power:
https://lkml.org/lkml/2019/1/31/259
Andrea has a longer reply there which might be far better than my commit messages. Please feel free to have a look too.
Thanks,
> > Jean > > [1] https://patchwork.kernel.org/cover/10743133/#22446425
-- Peter Xu
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