Re: [PATCH 14/17] mm/filemap: Restructure filemap_get_pages

[Christoph Hellwig <hch@lst.de>]

On Mon, Nov 02, 2020 at 06:43:09PM +0000, Matthew Wilcox (Oracle) wrote:
> Avoid a goto, and by the time we get to calling filemap_update_page(),
> we definitely have at least one page.

I find the error handling flow hard to follow and the existing but
heavily touched naming of the nr_got variable and the find_pages label
not helpful.  I'd do the following on top of this patch:

diff --git a/mm/filemap.c b/mm/filemap.c
index f16b1eb03bcad0..3ef73a58ce9456 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2344,51 +2344,54 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 	pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
 	pgoff_t last_index = (iocb->ki_pos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
 	struct page *page;
-	int nr_got, err = 0;
+	int nr_pages, err = 0;
 
 	nr = min_t(unsigned long, last_index - index, nr);
-find_page:
+retry:
 	if (fatal_signal_pending(current))
 		return -EINTR;
 
-	nr_got = mapping_get_read_thps(mapping, index, nr, pages);
-	if (!nr_got) {
+	nr_pages = mapping_get_read_thps(mapping, index, nr, pages);
+	if (!nr_pages) {
 		if (iocb->ki_flags & IOCB_NOIO)
 			return -EAGAIN;
 		page_cache_sync_readahead(mapping, ra, filp, index,
 				last_index - index);
-		nr_got = mapping_get_read_thps(mapping, index, nr, pages);
+		nr_pages = mapping_get_read_thps(mapping, index, nr, pages);
 	}
-	if (!nr_got) {
+	if (!nr_pages) {
 		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
 			return -EAGAIN;
 		pages[0] = filemap_create_page(filp, mapping,
 				iocb->ki_pos >> PAGE_SHIFT);
 		if (!pages[0])
-			goto find_page;
+			goto retry;
 		if (IS_ERR(pages[0]))
 			return PTR_ERR(pages[0]);
 		return 1;
 	}
 
-	page = pages[nr_got - 1];
-	if (PageReadahead(page))
+	page = pages[nr_pages - 1];
+	if (PageReadahead(page)) {
 		err = filemap_readahead(iocb, filp, mapping, page, last_index);
-	if (!err && !PageUptodate(page))
+		if (err)
+			goto error;
+	}
+	if (!PageUptodate(page)) {
 		err = filemap_update_page(iocb, mapping, iter, page,
-				nr_got == 1);
+				nr_pages == 1);
+		if (err)
+			goto error;
+	}
 
-	if (err)
-		nr_got--;
-	if (likely(nr_got))
-		return nr_got;
-	if (err < 0)
-		return err;
-	err = 0;
-	/*
-	 * No pages and no error means we raced and should retry:
-	 */
-	goto find_page;
+	return nr_pages;
+
+error:
+	if (nr_pages > 1)
+		return nr_pages - 1;
+	if (err == AOP_TRUNCATED_PAGE)
+		goto retry;
+	return err;
 }
 
 /**