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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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