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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