On 8/5/26 08:40, Shivank Garg wrote: > Implement kvm_gmem_migrate_folio() using filemap_migrate_folio() for > non-confidential VMs but don't enable it yet. guest_memfd mappings > are still marked unmovable, so MM never calls it. > > MM invokes ->migrate_folio() callback with folios already locked and > acquires the invalidate lock afterwards. kvm_gmem_punch_hole() takes > the invalidate lock before locking folios during truncation, so > taking invalidate lock in migration callback can cause an ABBA-deadlock > situation. Use filemap_invalidate_trylock_shared() to avoid this. > > A subsequent patch adds an opt-in flag to change the mapping to > movable and enable migration. Non-confidential VMs can use > host-side copy via folio_mc_copy(), whereas confidential VMs will > require firmware-assisted copying before they can opt in. > > Signed-off-by: Shivank Garg <[email protected]> > --- > virt/kvm/guest_memfd.c | 36 ++++++++++++++++++++++++++++++++++-- > 1 file changed, 34 insertions(+), 2 deletions(-) > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 169f75f95433..583341d593c9 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -488,13 +488,45 @@ static struct file_operations kvm_gmem_fops = { > .fallocate = kvm_gmem_fallocate, > }; > > +#ifdef CONFIG_MIGRATION > static int kvm_gmem_migrate_folio(struct address_space *mapping, > struct folio *dst, struct folio *src, > enum migrate_mode mode) > { > - WARN_ON_ONCE(1); > - return -EINVAL; > + struct inode *inode = mapping->host; > + pgoff_t start, end; > + int ret; > + > + /* > + * Migration invokes ->migrate_folio() while holding the folio lock. > + * Use a non-blocking trylock to avoid inverting the lock order with > + * truncation, which takes the invalidate lock before locking the > + * folios. > + */ > + if (!filemap_invalidate_trylock_shared(mapping)) > + return -EAGAIN; > + > + start = src->index; > + end = start + folio_nr_pages(src); > + > + kvm_gmem_invalidate_start(inode, start, end); > + > + /* > + * For non-confidential guests the folio is host-readable, so > + * filemap_migrate_folio() can copy the contents itself via > + * folio_mc_copy(). > + * For confidential guests, this would need firmware assistance. > + */
I think this is all more closely related to mapping_inaccessible(). If accessible, this is just trivially true that we can support this. So I would not comment on what to do for confidential VMs for now, and limit it only to the shared-only case where pages are accessible. -- Cheers, David
