9p: Copy local writes to the cache when writing to the server

When writing to the server from v9fs_vfs_writepage(), copy the data to the
cache object too.

To make this possible, the cookie must have its active users count
incremented when the page is dirtied and kept incremented until we manage
to clean up all the pages.  This allows the writeback to take place after
the last file struct is released.

This is done by taking a use on the cookie in v9fs_set_page_dirty() if we
haven't already done so (controlled by the I_PINNING_FSCACHE_WB flag) and
dropping the pin in v9fs_write_inode() if __writeback_single_inode() clears
all the outstanding dirty pages (conveyed by the unpinned_fscache_wb flag
in the writeback_control struct).

Inode eviction must also clear the flag after truncating away all the
outstanding pages.

In the future this will be handled more gracefully by netfslib.

Changes
=======
ver #3:
 - Canonicalise the coherency data to make it endianness-independent.

ver #2:
 - Fix an unused-var warning due to CONFIG_9P_FSCACHE=n[1].

Signed-off-by: David Howells <[email protected]>
Acked-by: Jeff Layton <[email protected]>
Tested-by: Dominique Martinet <[email protected]>
cc: Eric Van Hensbergen <[email protected]>
cc: Latchesar Ionkov <[email protected]>
cc: [email protected]
cc: [email protected]
Link: https://lore.kernel.org/r/163819667027.215744.13815687931204222995.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906978015.143852.10646669694345706328.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967180760.1823006.5831751873616248910.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021574522.640689.13849966660182529125.stgit@warthog.procyon.org.uk/ # v4

fs/9p/vfs_addr.c

@@ -137,6 +137,7 @@ static void v9fs_vfs_readahead(struct readahead_control *ractl)
 static int v9fs_release_page(struct page *page, gfp_t gfp)
 {
 	struct folio *folio = page_folio(page);
+	struct inode *inode = folio_inode(folio);
 
 	if (folio_test_private(folio))
 		return 0;
@@ -147,6 +148,7 @@ static int v9fs_release_page(struct page *page, gfp_t gfp)
 		folio_wait_fscache(folio);
 	}
 #endif
+	fscache_note_page_release(v9fs_inode_cookie(V9FS_I(inode)));
 	return 1;
 }
 
@@ -165,10 +167,25 @@ static void v9fs_invalidate_page(struct page *page, unsigned int offset,
 	folio_wait_fscache(folio);
 }
 
+static void v9fs_write_to_cache_done(void *priv, ssize_t transferred_or_error,
+				     bool was_async)
+{
+	struct v9fs_inode *v9inode = priv;
+	__le32 version;
+
+	if (IS_ERR_VALUE(transferred_or_error) &&
+	    transferred_or_error != -ENOBUFS) {
+		version = cpu_to_le32(v9inode->qid.version);
+		fscache_invalidate(v9fs_inode_cookie(v9inode), &version,
+				   i_size_read(&v9inode->vfs_inode), 0);
+	}
+}
+
 static int v9fs_vfs_write_folio_locked(struct folio *folio)
 {
 	struct inode *inode = folio_inode(folio);
 	struct v9fs_inode *v9inode = V9FS_I(inode);
+	struct fscache_cookie *cookie = v9fs_inode_cookie(v9inode);
 	loff_t start = folio_pos(folio);
 	loff_t i_size = i_size_read(inode);
 	struct iov_iter from;
@@ -185,10 +202,21 @@ static int v9fs_vfs_write_folio_locked(struct folio *folio)
 	/* We should have writeback_fid always set */
 	BUG_ON(!v9inode->writeback_fid);
 
+	folio_wait_fscache(folio);
 	folio_start_writeback(folio);
 
 	p9_client_write(v9inode->writeback_fid, start, &from, &err);
 
+	if (err == 0 &&
+	    fscache_cookie_enabled(cookie) &&
+	    test_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags)) {
+		folio_start_fscache(folio);
+		fscache_write_to_cache(v9fs_inode_cookie(v9inode),
+				       folio_mapping(folio), start, len, i_size,
+				       v9fs_write_to_cache_done, v9inode,
+				       true);
+	}
+
 	folio_end_writeback(folio);
 	return err;
 }
@@ -307,6 +335,7 @@ static int v9fs_write_end(struct file *filp, struct address_space *mapping,
 	loff_t last_pos = pos + copied;
 	struct folio *folio = page_folio(subpage);
 	struct inode *inode = mapping->host;
+	struct v9fs_inode *v9inode = V9FS_I(inode);
 
 	p9_debug(P9_DEBUG_VFS, "filp %p, mapping %p\n", filp, mapping);
 
@@ -326,6 +355,7 @@ static int v9fs_write_end(struct file *filp, struct address_space *mapping,
 	if (last_pos > inode->i_size) {
 		inode_add_bytes(inode, last_pos - inode->i_size);
 		i_size_write(inode, last_pos);
+		fscache_update_cookie(v9fs_inode_cookie(v9inode), NULL, &last_pos);
 	}
 	folio_mark_dirty(folio);
 out:
@@ -335,11 +365,25 @@ static int v9fs_write_end(struct file *filp, struct address_space *mapping,
 	return copied;
 }
 
+#ifdef CONFIG_9P_FSCACHE
+/*
+ * Mark a page as having been made dirty and thus needing writeback.  We also
+ * need to pin the cache object to write back to.
+ */
+static int v9fs_set_page_dirty(struct page *page)
+{
+	struct v9fs_inode *v9inode = V9FS_I(page->mapping->host);
+
+	return fscache_set_page_dirty(page, v9fs_inode_cookie(v9inode));
+}
+#else
+#define v9fs_set_page_dirty __set_page_dirty_nobuffers
+#endif
 
 const struct address_space_operations v9fs_addr_operations = {
 	.readpage = v9fs_vfs_readpage,
 	.readahead = v9fs_vfs_readahead,
-	.set_page_dirty = __set_page_dirty_nobuffers,
+	.set_page_dirty = v9fs_set_page_dirty,
 	.writepage = v9fs_vfs_writepage,
 	.write_begin = v9fs_write_begin,
 	.write_end = v9fs_write_end,

fs/9p/vfs_inode.c

@@ -380,8 +380,12 @@ struct inode *v9fs_get_inode(struct super_block *sb, umode_t mode, dev_t rdev)
 void v9fs_evict_inode(struct inode *inode)
 {
 	struct v9fs_inode *v9inode = V9FS_I(inode);
+	__le32 version;
 
 	truncate_inode_pages_final(&inode->i_data);
+	version = cpu_to_le32(v9inode->qid.version);
+	fscache_clear_inode_writeback(v9fs_inode_cookie(v9inode), inode,
+				      &version);
 	clear_inode(inode);
 	filemap_fdatawrite(&inode->i_data);
 

fs/9p/vfs_super.c

@@ -20,6 +20,7 @@
 #include <linux/slab.h>
 #include <linux/statfs.h>
 #include <linux/magic.h>
+#include <linux/fscache.h>
 #include <net/9p/9p.h>
 #include <net/9p/client.h>
 
@@ -309,6 +310,7 @@ static int v9fs_write_inode(struct inode *inode,
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 		return ret;
 	}
+	fscache_unpin_writeback(wbc, v9fs_inode_cookie(v9inode));
 	return 0;
 }
 
@@ -332,6 +334,7 @@ static int v9fs_write_inode_dotl(struct inode *inode,
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 		return ret;
 	}
+	fscache_unpin_writeback(wbc, v9fs_inode_cookie(v9inode));
 	return 0;
 }