afs: Fix to take ref on page when PG_private is set

Fix afs to take a ref on a page when it sets PG_private on it and to drop
the ref when removing the flag.

Note that in afs_write_begin(), a lot of the time, PG_private is already
set on a page to which we're going to add some data.  In such a case, we
leave the bit set and mustn't increment the page count.

As suggested by Matthew Wilcox, use attach/detach_page_private() where
possible.

Fixes: 31143d5d ("AFS: implement basic file write support")
Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

fs/afs/dir.c

@@ -281,8 +281,7 @@ static struct afs_read *afs_read_dir(struct afs_vnode *dvnode, struct key *key)
 			if (ret < 0)
 				goto error;
 
-			set_page_private(req->pages[i], 1);
-			SetPagePrivate(req->pages[i]);
+			attach_page_private(req->pages[i], (void *)1);
 			unlock_page(req->pages[i]);
 			i++;
 		} else {
@@ -1975,8 +1974,7 @@ static int afs_dir_releasepage(struct page *page, gfp_t gfp_flags)
 
 	_enter("{{%llx:%llu}[%lu]}", dvnode->fid.vid, dvnode->fid.vnode, page->index);
 
-	set_page_private(page, 0);
-	ClearPagePrivate(page);
+	detach_page_private(page);
 
 	/* The directory will need reloading. */
 	if (test_and_clear_bit(AFS_VNODE_DIR_VALID, &dvnode->flags))
@@ -2003,8 +2001,6 @@ static void afs_dir_invalidatepage(struct page *page, unsigned int offset,
 		afs_stat_v(dvnode, n_inval);
 
 	/* we clean up only if the entire page is being invalidated */
-	if (offset == 0 && length == PAGE_SIZE) {
-		set_page_private(page, 0);
-		ClearPagePrivate(page);
-	}
+	if (offset == 0 && length == PAGE_SIZE)
+		detach_page_private(page);
 }

fs/afs/dir_edit.c

@@ -243,10 +243,8 @@ void afs_edit_dir_add(struct afs_vnode *vnode,
 						   index, gfp);
 			if (!page)
 				goto error;
-			if (!PagePrivate(page)) {
-				set_page_private(page, 1);
-				SetPagePrivate(page);
-			}
+			if (!PagePrivate(page))
+				attach_page_private(page, (void *)1);
 			dir_page = kmap(page);
 		}
 

fs/afs/file.c

@@ -627,11 +627,9 @@ static void afs_invalidatepage(struct page *page, unsigned int offset,
 #endif
 
 		if (PagePrivate(page)) {
-			priv = page_private(page);
+			priv = (unsigned long)detach_page_private(page);
 			trace_afs_page_dirty(vnode, tracepoint_string("inval"),
 					     page->index, priv);
-			set_page_private(page, 0);
-			ClearPagePrivate(page);
 		}
 	}
 
@@ -661,11 +659,9 @@ static int afs_releasepage(struct page *page, gfp_t gfp_flags)
 #endif
 
 	if (PagePrivate(page)) {
-		priv = page_private(page);
+		priv = (unsigned long)detach_page_private(page);
 		trace_afs_page_dirty(vnode, tracepoint_string("rel"),
 				     page->index, priv);
-		set_page_private(page, 0);
-		ClearPagePrivate(page);
 	}
 
 	/* indicate that the page can be released */

fs/afs/write.c

@@ -151,8 +151,10 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 	priv |= f;
 	trace_afs_page_dirty(vnode, tracepoint_string("begin"),
 			     page->index, priv);
-	SetPagePrivate(page);
+	if (PagePrivate(page))
 		set_page_private(page, priv);
+	else
+		attach_page_private(page, (void *)priv);
 	_leave(" = 0");
 	return 0;
 
@@ -334,10 +336,9 @@ static void afs_pages_written_back(struct afs_vnode *vnode,
 		ASSERTCMP(pv.nr, ==, count);
 
 		for (loop = 0; loop < count; loop++) {
-			priv = page_private(pv.pages[loop]);
+			priv = (unsigned long)detach_page_private(pv.pages[loop]);
 			trace_afs_page_dirty(vnode, tracepoint_string("clear"),
 					     pv.pages[loop]->index, priv);
-			set_page_private(pv.pages[loop], 0);
 			end_page_writeback(pv.pages[loop]);
 		}
 		first += count;
@@ -863,8 +864,10 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
 	priv |= 0; /* From */
 	trace_afs_page_dirty(vnode, tracepoint_string("mkwrite"),
 			     vmf->page->index, priv);
-	SetPagePrivate(vmf->page);
+	if (PagePrivate(vmf->page))
 		set_page_private(vmf->page, priv);
+	else
+		attach_page_private(vmf->page, (void *)priv);
 	file_update_time(file);
 
 	sb_end_pagefault(inode->i_sb);
@@ -926,10 +929,9 @@ int afs_launder_page(struct page *page)
 		ret = afs_store_data(mapping, page->index, page->index, t, f, true);
 	}
 
+	priv = (unsigned long)detach_page_private(page);
 	trace_afs_page_dirty(vnode, tracepoint_string("laundered"),
 			     page->index, priv);
-	set_page_private(page, 0);
-	ClearPagePrivate(page);
 
 #ifdef CONFIG_AFS_FSCACHE
 	if (PageFsCache(page)) {