netfs, ceph: Partially revert "netfs: Replace PG_fscache by setting folio->private and marking dirty"

This partially reverts commit 2ff1e97.

In addition to reverting the removal of PG_private_2 wrangling from the
buffered read code[1][2], the removal of the waits for PG_private_2 from
netfs_release_folio() and netfs_invalidate_folio() need reverting too.

It also adds a wait into ceph_evict_inode() to wait for netfs read and
copy-to-cache ops to complete.

Fixes: 2ff1e97 ("netfs: Replace PG_fscache by setting folio->private and marking dirty")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/3575457.1722355300@warthog.procyon.org.uk [1]
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8e5ced7804cb9184c4a23f8054551240562a8eda [2]
Link: https://lore.kernel.org/r/20240814203850.2240469-2-dhowells@redhat.com
cc: Max Kellermann <max.kellermann@ionos.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Xiubo Li <xiubli@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: ceph-devel@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: dhowells

fs/ceph/inode.c

@@ -695,6 +695,7 @@ void ceph_evict_inode(struct inode *inode)
 
 	percpu_counter_dec(&mdsc->metric.total_inodes);
 
+	netfs_wait_for_outstanding_io(inode);
 	truncate_inode_pages_final(&inode->i_data);
 	if (inode->i_state & I_PINNING_NETFS_WB)
 		ceph_fscache_unuse_cookie(inode, true);

fs/netfs/misc.c

@@ -101,6 +101,8 @@ void netfs_invalidate_folio(struct folio *folio, size_t offset, size_t length)
 
 	_enter("{%lx},%zx,%zx", folio->index, offset, length);
 
+	folio_wait_private_2(folio); /* [DEPRECATED] */
+
 	if (!folio_test_private(folio))
 		return;
 
@@ -165,6 +167,11 @@ bool netfs_release_folio(struct folio *folio, gfp_t gfp)
 
 	if (folio_test_private(folio))
 		return false;
+	if (unlikely(folio_test_private_2(folio))) { /* [DEPRECATED] */
+		if (current_is_kswapd() || !(gfp & __GFP_FS))
+			return false;
+		folio_wait_private_2(folio);
+	}
 	fscache_note_page_release(netfs_i_cookie(ctx));
 	return true;
 }