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SubjectRe: [kvm-devel] [RFC 7/8]KVM: swap out guest pages
2007/7/25, Shaohua Li <shaoh.li@gmail.com>:
> 2007/7/24, Avi Kivity <avi@qumranet.com>:
> > Shaohua Li wrote:
> > > Make KVM guest pages be allocated dynamically and able to be swaped out.
> > >
> > > One issue: all inodes returned from anon_inode_getfd are shared,
> > > if one module changes field of the inode, other moduels might break.
> > > Should we introduce a new API to not share inode?
> > >
> > > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> > > ---
> > >
> > > +static int kvm_set_page_dirty(struct page *page)
> > > +{
> > > + if (!PageDirty(page))
> > > + SetPageDirty(page);
> > > + return 0;
> > > +}
> > > +
> > > +static int kvm_writepage(struct page *page, struct writeback_control *wbc)
> > > +{
> > > + struct address_space *mapping = page->mapping;
> > > + struct kvm *kvm = address_space_to_kvm(mapping);
> > > + int ret = 0;
> > > +
> > > + /*
> > > + * gfn_to_page is called with kvm->lock hold, which might invoke page
> > > + * reclaim. So the .writepage should check if we already hold the lock
> > > + * to avoid deadlock.
> > > + */
> > > + if (!mutex_trylock(&kvm->lock)) {
> > > + set_page_dirty(page);
> > > + return AOP_WRITEPAGE_ACTIVATE;
> > > + }
> > > +
> > > + /*
> > > + * We just zap vcpu 0's page table. For a SMP guest, we should zap all
> > > + * vcpus'. It's better shadow page table is per-vm.
> > > + */
> > > + if (PagePrivate(page))
> > > + kvm_mmu_zap_pagetbl(&kvm->vcpus[0], page->index);
> > > +
> > > + ret = kvm_move_to_swap(page);
> > > + if (ret) {
> > > + set_page_dirty(page);
> > > + goto out;
> > > + }
> > > + unlock_page(page);
> > > +out:
> > > + mutex_unlock(&kvm->lock);
> > > +
> > > + return ret;
> > > +}
> > > +
> > >
> >
> > Perhaps we can use this as a base for userspace-allocated memory. We
> > still have a kvm inode and address_space; but instead of calling
> > kvm_move_to_swap(), we use the memory slot and virtual address offset to
> > locate the underlying address_space and call that ->writepage().
> >
> > So:
> > kvm_writepage() removes any shadow page table references
> > the underlying ->writepage() does the work of paging to the underlying
> > store
> So write to a file, right? Yes, it can avoid use move to swap, and
> should be feasible.
Say you want to write guest pages out to file A of back store fs, in
kvm->writepage(), we could do:
1. lower_page = grap_cache_page(file A's mapping)
2. file A's ->prepare_write(lower_page)
3. copy kvm guest page to lower_page
4. file A's ->commit_write(lower_page)
then guest page can be freed. Just like the stack fs does. The
downside is step 1 needs allocate a new page.

Thanks,
Shaohua
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