Re: [RFC PATCH] mm: Add helpers for locked_vm

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2014-07-30 at 14:47 +0200, Peter Zijlstra wrote:
> On Wed, Jul 30, 2014 at 10:30:48PM +1000, Alexey Kardashevskiy wrote:
> > 
> > No, this is not my intention here. Here I only want to increment the counter.
> 
> Full and hard nack on that. It should always be tied to actual pages, we
> should not detach this and make it 'a number'.

But this is the only way. We *cannot* go through the whole per-page
locking logic every time the guest puts a translation into the IOMMU,
this will completely kill guest performances for pass-through devices.

Worse, for performances, because populating the iommu is a hypercall,
we want to do it in "real mode" (special MMU-off environment) where we
cannot rely on most normal kernel services such as normal locks, vmalloc
space isn't accessible etc...

So we don't have a choice. Either we let guests randomly pin arbitrary
amounts of system memory, or we have a way to predictively account for
the maximum that *can* be mapped/pinned in the iommu table to enable
the fast path.

Another problem with the mlock logic is that it doesn't refcount how
many time a page has been locked, while the guest can map a given page
multiple time in the iommu.

Ben.



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