linux-kernel - Re: mmu notifier #v14

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Date:	Sun, 27 Apr 2008 02:20:19 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Anthony Liguori <aliguori@...ibm.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, Robin Holt <holt@....com>,
	Jack Steiner <steiner@....com>,
	Christoph Lameter <clameter@....com>,
	akpm@...ux-foundation.org, Nick Piggin <npiggin@...e.de>,
	Steve Wise <swise@...ngridcomputing.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	kvm-devel@...ts.sourceforge.net, general@...ts.openfabrics.org,
	Hugh Dickins <hugh@...itas.com>,
	Chris Wright <chrisw@...hat.com>,
	Marcelo Tosatti <marcelo@...ck.org>
Subject: Re: mmu notifier #v14

On Sat, Apr 26, 2008 at 01:59:23PM -0500, Anthony Liguori wrote:
>> +static void kvm_unmap_spte(struct kvm *kvm, u64 *spte)
>> +{
>> +	struct page *page = pfn_to_page((*spte & PT64_BASE_ADDR_MASK) >> 
>> PAGE_SHIFT);
>> +	get_page(page);
>>   
>
> You should not assume a struct page exists for any given spte. Instead, use 
> kvm_get_pfn() and kvm_release_pfn_clean().

Last email from muli@ibm in my inbox argues it's useless to build rmap
on mmio regions, so the above is more efficient so put_page runs
directly on the page without going back and forth between spte -> pfn
-> page -> pfn -> page in a single function.

Certainly if we start building rmap on mmio regions we'll have to
change that.

> Perhaps I just have a weak stomach but I am uneasy having a function that 
> takes a lock on exit. I walked through the logic and it doesn't appear to 
> be wrong but it also is pretty clear that you could defer the acquisition 
> of the lock to the caller (in this case, kvm_mmu_pte_write) by moving the 
> update_pte assignment into kvm_mmu_pte_write.

I agree out_lock is an uncommon exit path, the problem is that the
code was buggy, and I tried to fix it with the smallest possible
change and that resulting in an out_lock. That section likely need a
refactoring, all those update_pte fields should be at least returned
by the function guess_....  but I tried to reduce the changes to make
the issue more readable, I didn't want to rewrite certain functions
just to take a spinlock a few instructions ahead.

> Worst case, you pass 4 more pointer arguments here and, take the spin lock, 
> and then depending on the result of mmu_guess_page_from_pte_write, update 
> vcpu->arch.update_pte.

Yes that was my same idea, but that's left for a later patch. Fixing
this bug mixed with the mmu notifier patch was perhaps excessive
already ;).

> Why move the destruction of the vm to the MMU notifier unregister hook? 
> Does anything else ever call mmu_notifier_unregister that would implicitly 
> destroy the VM?

mmu notifier ->release can run at anytime before the filehandle is
closed. ->release has to zap all sptes and freeze the mmu (hence all
vcpus) to prevent any further page fault. After ->release returns all
pages are freed (we'll never relay on the page pin to avoid the
rmap_remove put_page to be a relevant unpin event). So the idea is
that I wanted to maintain the same ordering of the current code in the
vm destroy event, I didn't want to leave a partially shutdown VM on
the vmlist. If the ordering is entirely irrelevant and the
kvm_arch_destroy_vm can run well before kvm_destroy_vm is called, then
I can avoid changes to kvm_main.c but I doubt.

I've done it in a way that archs not needing mmu notifiers like s390
can simply add the kvm_destroy_common_vm at the top of their
kvm_arch_destroy_vm. All others using mmu_notifiers have to invoke
kvm_destroy_common_vm in the ->release of the mmu notifiers.

This will ensure that everything will be ok regardless if exit_mmap is
called before/after exit_files, and it won't make a whole lot of
difference anymore, if the driver fd is pinned through vmas->vm_file
released in exit_mmap or through the task filedescriptors relased in
exit_files etc... Infact this allows to call mmu_notifier_unregister
at anytime later after the task has already been killed, without any
trouble (like if the mmu notifier owner isn't registering in
current->mm but some other tasks mm).
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