Re: [RFP-V2 0/3] Make mmu_notifier_invalidate_range_start able to sleep.

[Andrea Arcangeli <aarcange@redhat.com>]

On Tue, Feb 02, 2010 at 03:09:47AM -0500, Christoph Hellwig wrote:
> On Mon, Feb 01, 2010 at 10:01:45PM -0600, Robin Holt wrote:
> > XPMEM would like to utilize mmu_notifiers to track page table entry
> > changes of the segment and keep the attachment page table/tlb information
> > consistent.
> 
> Given that SGI just pushes XPMEM direclty into the distributions instead
> of adding it upstream I don't really see the relevance of these patches.

That will then prevent upstream modules to build against those
kernels. Not an huge issue, for a distro that's an ok compromise. My
real issue with mainline is that while XPMEM is ok to break and
corrupt memory if people uses XPMEM on top of shared mappings (instead
of anonymous ones) by making a two liner change to the userland app
opening xpmem device, but when next mmu notifier user comes and ask
for full scheduling across shared mapping too as it needs security and
not-trusted user can open /dev/xpmem (or whatever that device is
located), we'll have to undo this work and fix it the real way (with
config option MMU_NOTIFIER_SLEEPABLE=y). But if distro have to support
XPMEM in default kernels, this hack is better because it won't
slowdown the locking even if it leaves holes and corrupts memory when
XPMEM can be opened by luser. It really depends if the user having
access to XPMEM device is malicious, if we know it's not (assume
avatar distributed rendering in closed environment or whatever) this
again is an ok hack.

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