[PATCH] add AOP_TRUNCATED_PAGE, prepend AOP_ to WRITEPAGE_ACTIVATE

readpage(), prepare_write(), and commit_write() callers are updated to
understand the special return code AOP_TRUNCATED_PAGE in the style of
writepage() and WRITEPAGE_ACTIVATE.  AOP_TRUNCATED_PAGE tells the caller that
the callee has unlocked the page and that the operation should be tried again
with a new page.  OCFS2 uses this to detect and work around a lock inversion in
its aop methods.  There should be no change in behaviour for methods that don't
return AOP_TRUNCATED_PAGE.

WRITEPAGE_ACTIVATE is also prepended with AOP_ for consistency and they are
made enums so that kerneldoc can be used to document their semantics.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

4 files changed, 61 insertions, 31 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 33a28bf..6e1d08a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -831,8 +831,13 @@ readpage:
 		/* Start the actual read. The read will unlock the page. */
 		error = mapping->a_ops->readpage(filp, page);
 
-		if (unlikely(error))
+		if (unlikely(error)) {
+			if (error == AOP_TRUNCATED_PAGE) {
+				page_cache_release(page);
+				goto find_page;
+			}
 			goto readpage_error;
+		}
 
 		if (!PageUptodate(page)) {
 			lock_page(page);
@@ -1152,26 +1157,24 @@ static int fastcall page_cache_read(struct file * file, unsigned long offset)
 {
 	struct address_space *mapping = file->f_mapping;
 	struct page *page; 
-	int error;
+	int ret;
 
-	page = page_cache_alloc_cold(mapping);
-	if (!page)
-		return -ENOMEM;
+	do {
+		page = page_cache_alloc_cold(mapping);
+		if (!page)
+			return -ENOMEM;
+
+		ret = add_to_page_cache_lru(page, mapping, offset, GFP_KERNEL);
+		if (ret == 0)
+			ret = mapping->a_ops->readpage(file, page);
+		else if (ret == -EEXIST)
+			ret = 0; /* losing race to add is OK */
 
-	error = add_to_page_cache_lru(page, mapping, offset, GFP_KERNEL);
-	if (!error) {
-		error = mapping->a_ops->readpage(file, page);
 		page_cache_release(page);
-		return error;
-	}
 
-	/*
-	 * We arrive here in the unlikely event that someone 
-	 * raced with us and added our page to the cache first
-	 * or we are out of memory for radix-tree nodes.
-	 */
-	page_cache_release(page);
-	return error == -EEXIST ? 0 : error;
+	} while (ret == AOP_TRUNCATED_PAGE);
+		
+	return ret;
 }
 
 #define MMAP_LOTSAMISS  (100)
@@ -1331,10 +1334,14 @@ page_not_uptodate:
 		goto success;
 	}
 
-	if (!mapping->a_ops->readpage(file, page)) {
+	error = mapping->a_ops->readpage(file, page);
+	if (!error) {
 		wait_on_page_locked(page);
 		if (PageUptodate(page))
 			goto success;
+	} else if (error == AOP_TRUNCATED_PAGE) {
+		page_cache_release(page);
+		goto retry_find;
 	}
 
 	/*
@@ -1358,10 +1365,14 @@ page_not_uptodate:
 		goto success;
 	}
 	ClearPageError(page);
-	if (!mapping->a_ops->readpage(file, page)) {
+	error = mapping->a_ops->readpage(file, page);
+	if (!error) {
 		wait_on_page_locked(page);
 		if (PageUptodate(page))
 			goto success;
+	} else if (error == AOP_TRUNCATED_PAGE) {
+		page_cache_release(page);
+		goto retry_find;
 	}
 
 	/*
@@ -1444,10 +1455,14 @@ page_not_uptodate:
 		goto success;
 	}
 
-	if (!mapping->a_ops->readpage(file, page)) {
+	error = mapping->a_ops->readpage(file, page);
+	if (!error) {
 		wait_on_page_locked(page);
 		if (PageUptodate(page))
 			goto success;
+	} else if (error == AOP_TRUNCATED_PAGE) {
+		page_cache_release(page);
+		goto retry_find;
 	}
 
 	/*
@@ -1470,10 +1485,14 @@ page_not_uptodate:
 	}
 
 	ClearPageError(page);
-	if (!mapping->a_ops->readpage(file, page)) {
+	error = mapping->a_ops->readpage(file, page);
+	if (!error) {
 		wait_on_page_locked(page);
 		if (PageUptodate(page))
 			goto success;
+	} else if (error == AOP_TRUNCATED_PAGE) {
+		page_cache_release(page);
+		goto retry_find;
 	}
 
 	/*
@@ -1934,12 +1953,16 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 		status = a_ops->prepare_write(file, page, offset, offset+bytes);
 		if (unlikely(status)) {
 			loff_t isize = i_size_read(inode);
+
+			if (status != AOP_TRUNCATED_PAGE)
+				unlock_page(page);
+			page_cache_release(page);
+			if (status == AOP_TRUNCATED_PAGE)
+				continue;
 			/*
 			 * prepare_write() may have instantiated a few blocks
 			 * outside i_size.  Trim these off again.
 			 */
-			unlock_page(page);
-			page_cache_release(page);
 			if (pos + bytes > isize)
 				vmtruncate(inode, isize);
 			break;
@@ -1952,6 +1975,10 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 						cur_iov, iov_base, bytes);
 		flush_dcache_page(page);
 		status = a_ops->commit_write(file, page, offset, offset+bytes);
+		if (status == AOP_TRUNCATED_PAGE) {
+			page_cache_release(page);
+			continue;
+		}
 		if (likely(copied > 0)) {
 			if (!status)
 				status = copied;
diff --git a/mm/readahead.c b/mm/readahead.c
index 72e7adb..8d6eeaa 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -158,7 +158,7 @@ static int read_pages(struct address_space *mapping, struct file *filp,
 {
 	unsigned page_idx;
 	struct pagevec lru_pvec;
-	int ret = 0;
+	int ret;
 
 	if (mapping->a_ops->readpages) {
 		ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
@@ -171,14 +171,17 @@ static int read_pages(struct address_space *mapping, struct file *filp,
 		list_del(&page->lru);
 		if (!add_to_page_cache(page, mapping,
 					page->index, GFP_KERNEL)) {
-			mapping->a_ops->readpage(filp, page);
-			if (!pagevec_add(&lru_pvec, page))
-				__pagevec_lru_add(&lru_pvec);
-		} else {
-			page_cache_release(page);
+			ret = mapping->a_ops->readpage(filp, page);
+			if (ret != AOP_TRUNCATED_PAGE) {
+				if (!pagevec_add(&lru_pvec, page))
+					__pagevec_lru_add(&lru_pvec);
+				continue;
+			} /* else fall through to release */
 		}
+		page_cache_release(page);
 	}
 	pagevec_lru_add(&lru_pvec);
+	ret = 0;
 out:
 	return ret;
 }
diff --git a/mm/shmem.c b/mm/shmem.c
index dc25565..d9fc277 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -855,7 +855,7 @@ unlock:
 	swap_free(swap);
 redirty:
 	set_page_dirty(page);
-	return WRITEPAGE_ACTIVATE;	/* Return with the page locked */
+	return AOP_WRITEPAGE_ACTIVATE;	/* Return with the page locked */
 }
 
 #ifdef CONFIG_NUMA
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b0cd81c..795a050 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -367,7 +367,7 @@ static pageout_t pageout(struct page *page, struct address_space *mapping)
 		res = mapping->a_ops->writepage(page, &wbc);
 		if (res < 0)
 			handle_write_error(mapping, page, res);
-		if (res == WRITEPAGE_ACTIVATE) {
+		if (res == AOP_WRITEPAGE_ACTIVATE) {
 			ClearPageReclaim(page);
 			return PAGE_ACTIVATE;
 		}