On Tue, Mar 18, 2025 at 9:20 AM Fuad Tabba <tabba@xxxxxxxxxx> wrote:
> ...
> +/*
> + * Callback function for __folio_put(), i.e., called once all references by the
> + * host to the folio have been dropped. This allows gmem to transition the state
> + * of the folio to shared with the guest, and allows the hypervisor to continue
> + * transitioning its state to private, since the host cannot attempt to access
> + * it anymore.
> + */
> void kvm_gmem_handle_folio_put(struct folio *folio)
> {
> - WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
> + struct address_space *mapping;
> + struct xarray *shared_offsets;
> + struct inode *inode;
> + pgoff_t index;
> + void *xval;
> +
> + mapping = folio->mapping;
> + if (WARN_ON_ONCE(!mapping))
> + return;
> +
> + inode = mapping->host;
> + index = folio->index;
> + shared_offsets = &kvm_gmem_private(inode)->shared_offsets;
> + xval = xa_mk_value(KVM_GMEM_GUEST_SHARED);
> +
> + filemap_invalidate_lock(inode->i_mapping);
As discussed in the guest_memfd upstream, folio_put can happen from
atomic context [1], so we need a way to either defer the work outside
kvm_gmem_handle_folio_put() (which is very likely needed to handle
hugepages and merge operation) or ensure to execute the logic using
synchronization primitives that will not sleep.
[1] https://elixir.bootlin.com/linux/v6.14-rc6/source/include/linux/mm.h#L1483
> + folio_lock(folio);
> + kvm_gmem_restore_pending_folio(folio, inode);
> + folio_unlock(folio);
> + WARN_ON_ONCE(xa_err(xa_store(shared_offsets, index, xval, GFP_KERNEL)));
> + filemap_invalidate_unlock(inode->i_mapping);
> }
> EXPORT_SYMBOL_GPL(kvm_gmem_handle_folio_put);
>
> --
> 2.49.0.rc1.451.g8f38331e32-goog
>
