Re: [PATCH] kvm swapping with mmu notifiers + age_page

[Andrea Arcangeli <andrea-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

On Tue, Jan 22, 2008 at 04:08:16PM +0200, Avi Kivity wrote:
> Andrea Arcangeli wrote:
>> This is the same as before but it uses the age_page callback to
>> prevent the guest OS working set to be swapped out. It works well here
>> so far. This depends on the memslot locking with mmu lock patch and on
>> the mmu notifiers #v3 patch that I'll post in CC with linux-mm shortly
>> that implements the age_page callback and that changes follow_page to
>> set the young bit in the pte instead of setting the referenced bit (so
>> the age_page will be called again later when the VM clears the young
>> bit).
>>
>>  +static void unmap_spte(struct kvm *kvm, u64 *spte)
>> +{
>> +	struct page *page = pfn_to_page((*spte & PT64_BASE_ADDR_MASK) >> 
>> PAGE_SHIFT);
>> +	get_page(page);
>> +	rmap_remove(kvm, spte);
>> +	set_shadow_pte(spte, shadow_trap_nonpresent_pte);
>> +	kvm_flush_remote_tlbs(kvm);
>> +	__free_page(page);
>> +}
>>   
>
> Why is get_page()/__free_page() needed here? Isn't kvm_release_page_*() 
> sufficient?

The other-cpus-tlb have to be flushed _before_ the page is visible in
the host kernel freelist, otherwise other host-cpus with tlbs still
mapping the page with write-access would be able to modify the page
even after it's queued in the freelist. The mmu_notifier are called in
places like munmap where the __free_page will not be a put_page but a
real __free_page. Furthermore kvm_release_page_ aren't calling
__free_page but put_page that would leak ram in those paths (mostly
invalidate_range). I'd rather not depend on the mmu_notifiers always
being invoked with an additional reference count on the page (in
addition to the spte reference count). The ->invalidate_* methods
might be the ones that put the page in the freelist.

-------------------------------------------------------------------------
This SF.net email is sponsored by: Microsoft
Defy all challenges. Microsoft(R) Visual Studio 2008.
http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/