On Fri, Jan 17, 2025 at 04:29:51PM +0000, Fuad Tabba wrote:
> +/*
> + * Marks the range [start, end) as not mappable by the host. If the host doesn't
> + * have any references to a particular folio, then that folio is marked as
> + * mappable by the guest.
> + *
> + * However, if the host still has references to the folio, then the folio is
> + * marked and not mappable by anyone. Marking it is not mappable allows it to
> + * drain all references from the host, and to ensure that the hypervisor does
> + * not transition the folio to private, since the host still might access it.
> + *
> + * Usually called when guest unshares memory with the host.
> + */
> +static int gmem_clear_mappable(struct inode *inode, pgoff_t start, pgoff_t end)
> +{
> + struct xarray *mappable_offsets = &kvm_gmem_private(inode)->mappable_offsets;
> + void *xval_guest = xa_mk_value(KVM_GMEM_GUEST_MAPPABLE);
> + void *xval_none = xa_mk_value(KVM_GMEM_NONE_MAPPABLE);
> + pgoff_t i;
> + int r = 0;
> +
> + filemap_invalidate_lock(inode->i_mapping);
> + for (i = start; i < end; i++) {
> + struct folio *folio;
> + int refcount = 0;
> +
> + folio = filemap_lock_folio(inode->i_mapping, i);
> + if (!IS_ERR(folio)) {
> + refcount = folio_ref_count(folio);
> + } else {
> + r = PTR_ERR(folio);
> + if (WARN_ON_ONCE(r != -ENOENT))
> + break;
> +
> + folio = NULL;
> + }
> +
> + /* +1 references are expected because of filemap_lock_folio(). */
> + if (folio && refcount > folio_nr_pages(folio) + 1) {
Looks racy.
What prevent anybody from obtaining a reference just after check?
Lock on folio doesn't stop random filemap_get_entry() from elevating the
refcount.
folio_ref_freeze() might be required.
> + /*
> + * Outstanding references, the folio cannot be faulted
> + * in by anyone until they're dropped.
> + */
> + r = xa_err(xa_store(mappable_offsets, i, xval_none, GFP_KERNEL));
> + } else {
> + /*
> + * No outstanding references. Transition the folio to
> + * guest mappable immediately.
> + */
> + r = xa_err(xa_store(mappable_offsets, i, xval_guest, GFP_KERNEL));
> + }
> +
> + if (folio) {
> + folio_unlock(folio);
> + folio_put(folio);
> + }
> +
> + if (WARN_ON_ONCE(r))
> + break;
> + }
> + filemap_invalidate_unlock(inode->i_mapping);
> +
> + return r;
> +}
--
Kiryl Shutsemau / Kirill A. Shutemov
