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

[Jerome Glisse <jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Tue, Oct 15, 2019 at 03:12:27PM -0300, Jason Gunthorpe wrote:
> 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.
> 
> This new arrangment plays nicely with the !blockable mode for
> OOM. Scanning the interval tree is done such that the intersection test
> will always succeed, and since there is no invalidate_range_end exposed to
> drivers the scheme safely allows multiple drivers to be subscribed.
> 
> Four places are converted as an example of how the new API is used.
> Four are left for future patches:
>  - i915_gem has complex locking around destruction of a registration,
>    needs more study
>  - hfi1 (2nd user) needs access to the rbtree
>  - scif_dma has a complicated logic flow
>  - vhost's mmu notifiers are already being rewritten
> 
> This is still being tested, but I figured to send it to start getting help
> from the xen, amd and hfi drivers which I cannot test here.

It might be a good oportunity to also switch those users to
hmm_range_fault() instead of GUP as GUP is pointless for those
users. In fact the GUP is an impediment to normal mm operations.

I will test on nouveau.

Cheers,
Jérôme

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