[RFC PATCH] mm, netfs: Provide a means of invalidation without using launder_folio

[David Howells <dhowells@redhat.com>]

Implement a replacement for launder_folio[1].  The key feature of
invalidate_inode_pages2() is that it locks each folio individually, unmaps
it to prevent mmap'd accesses interfering and calls the ->launder_folio()
address_space op to flush it.  This has problems: firstly, each folio is
written individually as one or more small writes; secondly, adjacent folios
cannot be added so easily into the laundry; thirdly, it's yet another op to
implement.

Here's a bit of a hacked together solution which should probably be moved
to mm/:

Use the mmap lock to cause future faulting to wait, then unmap all the
folios if we have mmaps, then, conditionally, use ->writepages() to flush
any dirty data back and then discard all pages.  The caller needs to hold a
lock to prevent ->write_iter() getting underfoot.

Note that this does not prevent ->read_iter() from accessing the file
whilst we do this since that may operate without locking.

We also have the writeback_control available and so have the opportunity to
set a flag in it to tell the filesystem that we're doing an invalidation.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Miklos Szeredi <miklos@szeredi.hu>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Christian Brauner <brauner@kernel.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-mm@kvack.org
cc: linux-fsdevel@vger.kernel.org
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: devel@lists.orangefs.org
Link: https://lore.kernel.org/r/1668172.1709764777@warthog.procyon.org.uk/ [1]
---
 fs/netfs/misc.c       |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |    3 ++
 mm/memory.c           |    3 +-
 3 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
index bc1fc54fb724..774ce825fbec 100644
--- a/fs/netfs/misc.c
+++ b/fs/netfs/misc.c
@@ -250,3 +250,59 @@ bool netfs_release_folio(struct folio *folio, gfp_t gfp)
 	return true;
 }
 EXPORT_SYMBOL(netfs_release_folio);
+
+extern void unmap_mapping_range_tree(struct rb_root_cached *root,
+				     pgoff_t first_index,
+				     pgoff_t last_index,
+				     struct zap_details *details);
+
+/**
+ * netfs_invalidate_inode - Invalidate/forcibly write back an inode's pagecache
+ * @inode: The inode to flush
+ * @flush: Set to write back rather than simply invalidate.
+ *
+ * Invalidate all the folios on an inode, possibly writing them back first.
+ * Whilst the operation is undertaken, the mmap lock is held to prevent
+ * ->fault() from reinstalling the folios.  The caller must hold a lock on the
+ * inode sufficient to prevent ->write_iter() from dirtying more folios.
+ */
+int netfs_invalidate_inode(struct inode *inode, bool flush)
+{
+	struct address_space *mapping = inode->i_mapping;
+
+	if (!mapping || !mapping->nrpages)
+		goto out;
+
+	/* Prevent folios from being faulted in. */
+	i_mmap_lock_write(mapping);
+
+	if (!mapping->nrpages)
+		goto unlock;
+
+	/* Assume there are probably PTEs only if there are mmaps. */
+	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root))) {
+		struct zap_details details = { };
+
+		unmap_mapping_range_tree(&mapping->i_mmap, 0, LLONG_MAX, &details);
+	}
+
+	/* Write back the data if we're asked to. */
+	if (flush) {
+		struct writeback_control wbc = {
+			.sync_mode	= WB_SYNC_ALL,
+			.nr_to_write	= LONG_MAX,
+			.range_start	= 0,
+			.range_end	= LLONG_MAX,
+		};
+
+		filemap_fdatawrite_wbc(mapping, &wbc);
+	}
+
+	/* Wait for writeback to complete on all folios and discard. */
+	truncate_inode_pages_range(mapping, 0, LLONG_MAX);
+
+unlock:
+	i_mmap_unlock_write(mapping);
+out:
+	return filemap_check_errors(mapping);
+}
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 298552f5122c..40dc34ee291d 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -400,6 +400,9 @@ ssize_t netfs_buffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *fr
 ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from);
 ssize_t netfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from);
 
+/* High-level invalidation API */
+int netfs_invalidate_inode(struct inode *inode, bool flush);
+
 /* Address operations API */
 struct readahead_control;
 void netfs_readahead(struct readahead_control *);
diff --git a/mm/memory.c b/mm/memory.c
index f2bc6dd15eb8..106f32c7d7fb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3665,7 +3665,7 @@ static void unmap_mapping_range_vma(struct vm_area_struct *vma,
 	zap_page_range_single(vma, start_addr, end_addr - start_addr, details);
 }
 
-static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
+inline void unmap_mapping_range_tree(struct rb_root_cached *root,
 					    pgoff_t first_index,
 					    pgoff_t last_index,
 					    struct zap_details *details)
@@ -3685,6 +3685,7 @@ static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
 				details);
 	}
 }
+EXPORT_SYMBOL_GPL(unmap_mapping_range_tree);
 
 /**
  * unmap_mapping_folio() - Unmap single folio from processes.