Messages in this thread | | | Subject | Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers | From | Ralph Campbell <> | Date | Thu, 16 Jan 2020 12:16:30 -0800 |
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On 1/16/20 8:00 AM, Jason Gunthorpe wrote: > On Wed, Jan 15, 2020 at 02:09:47PM -0800, Ralph Campbell wrote: > >> I don't understand the lifetime/membership issue. The driver is the only thing >> that allocates, inserts, or removes struct mmu_interval_notifier and thus >> completely controls the lifetime. > > If the returned value is on the defered list it could be freed at any > moment. The existing locks do not prevent it. > >>>> + ret = nouveau_svmm_interval_find(svmm, &range); >>>> + if (ret) { >>>> + up_read(&mm->mmap_sem); >>>> + return ret; >>>> + } >>>> + range.notifier_seq = mmu_interval_read_begin(range.notifier); >>>> ret = hmm_range_fault(&range, 0); >>>> up_read(&mm->mmap_sem); >>>> if (ret <= 0) { >>> >>> I'm still not sure this is a better approach than what ODP does. It >>> looks very expensive on the fault path.. >>> >>> Jason >>> >> >> ODP doesn't have this problem because users have to call ib_reg_mr() >> before any I/O can happen to the process address space. > > ODP supports a single 'full VA' call at process startup, just like > these cases. > >> That is when mmu_interval_notifier_insert() / >> mmu_interval_notifier_remove() can be called and the driver doesn't >> have to worry about the interval changing sizes or being removed >> while I/O is happening. > > No, for the 'ODP full process VA' (aka implicit ODP) mode it > dynamically maintains a list of intervals. ODP chooses the align the > dynamic intervals to it's HW page table levels, and not to SW VMAs. > This is much simpler to manage and faster to fault, at the cost of > capturing more VA for invalidations which have to be probed against > the HW shadow PTEs. > >> It isn't that expensive, there is an extra driver lock/unlock as >> part of the lookup and possibly a find_vma() and kmalloc(GFP_ATOMIC) >> for new intervals. Also, the deferred interval updates for munmap(). >> Compared to the cost of updating PTEs in the device and GPU fault >> handling, this is minimal overhead. > > Well, compared to ODP which does a single xa lookup with no lock to > find its interval, this looks very expensive and not parallel. > > I think if there is merit in having ranges cover the vmas and track > changes then there is probably merit in having the core code provide > much of that logic, not the driver. > > But it would be interesting to see some kind of analysis on the two > methods to decide if the complexity is worthwhile. > > Jason >
Can you point me to the latest ODP code? Seems like my understanding is quite off.
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