Re: [PATCH hmm 00/15] Consolidate the mmu notifier interval_tree and locking

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 16.10.19 um 18:04 schrieb Jason Gunthorpe:
> On Wed, Oct 16, 2019 at 10:58:02AM +0200, Christian König wrote:
>> Am 15.10.19 um 20:12 schrieb Jason Gunthorpe:
>>> From: Jason Gunthorpe <jgg@mellanox.com>
>>>
>>> 8 of the mmu_notifier using drivers (i915_gem, radeon_mn, umem_odp, hfi1,
>>> scif_dma, vhost, gntdev, hmm) drivers are using a common pattern where
>>> they only use invalidate_range_start/end and immediately check the
>>> invalidating range against some driver data structure to tell if the
>>> driver is interested. Half of them use an interval_tree, the others are
>>> simple linear search lists.
>>>
>>> Of the ones I checked they largely seem to have various kinds of races,
>>> bugs and poor implementation. This is a result of the complexity in how
>>> the notifier interacts with get_user_pages(). It is extremely difficult to
>>> use it correctly.
>>>
>>> Consolidate all of this code together into the core mmu_notifier and
>>> provide a locking scheme similar to hmm_mirror that allows the user to
>>> safely use get_user_pages() and reliably know if the page list still
>>> matches the mm.
>> That sounds really good, but could you outline for a moment how that is
>> archived?
> It uses the same basic scheme as hmm and rdma odp, outlined in the
> revisions to hmm.rst later on.
>
> Basically,
>
>   seq = mmu_range_read_begin(&mrn);
>
>   // This is a speculative region
>   .. get_user_pages()/hmm_range_fault() ..

How do we enforce that this get_user_pages()/hmm_range_fault() doesn't 
see outdated page table information?

In other words how the the following race prevented:

CPU A CPU B
invalidate_range_start()
       mmu_range_read_begin()
       get_user_pages()/hmm_range_fault()
Updating the ptes
invalidate_range_end()


I mean get_user_pages() tries to circumvent this issue by grabbing a 
reference to the pages in question, but that isn't sufficient for the 
SVM use case.

That's the reason why we had this horrible solution with a r/w lock and 
a linked list of BOs in an interval tree.

Regards,
Christian.

>   // Result cannot be derferenced
>
>   take_lock(driver->update);
>   if (mmu_range_read_retry(&mrn, range.notifier_seq) {
>      // collision! The results are not correct
>      goto again
>   }
>
>   // no collision, and now under lock. Now we can de-reference the pages/etc
>   // program HW
>   // Now the invalidate callback is responsible to synchronize against changes
>   unlock(driver->update)
>
> Basically, anything that was using hmm_mirror correctly transisions
> over fairly trivially, just with the modification to store a sequence
> number to close that race described in the hmm commit.
>
> For something like AMD gpu I expect it to transition to use dma_fence
> from the notifier for coherency right before it unlocks driver->update.
>
> Jason
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