Re: [PATCH 0/3] Enable clients to schedule in mmu_notifier methods

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 05, 2012 at 04:35:49PM +0300, Haggai Eran wrote:
> The following short patch series completes the support for allowing clients to
> sleep in mmu notifiers (specifically in invalidate_page and
> invalidate_range_start/end), adding on the work done by Andrea Arcangeli and
> Sagi Grimberg in http://marc.info/?l=linux-mm&m=133113297028676&w=3
> 
> This patchset is a preliminary step towards on-demand paging design to be
> added to the Infiniband stack. Our goal is to avoid pinning pages in
> memory regions registered for IB communication, so we need to get
> notifications for invalidations on such memory regions, and stop the hardware
> from continuing its access to the invalidated pages. The hardware operation
> that flushes the page tables can block, so we need to sleep until the hardware
> is guaranteed not to access these pages anymore.

Since people have been asking about the need for on demand paging in
devices: this can be useful for KVM where we sometimes want to let guest
directly (bypassing the hypervisor) control a virtual function of a PCI
device (prevented by an iommu from accessing host memory).

Currently this means host needs to pin all guest memory that *might* be used
by this virtual function which breaks setups with memory overcommit; at
the moment we can address this by means of ballooning - cooperative memory
management - but this has some obvious problems: for example, to get some
memory out of a low priority guest it needs to run so the balloon can
get inflated. By comparison on demand paging would not require
guest cooperation.

The problem is not specific to Infiniband; addressing it by on demand
paging does require hardware and host driver support though. If
Infiniband drivers happen to be the first to implement it more
power to them.

-- 
MST

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