linux-kernel - [PATCH v8 12/25] mm: Move end_index check out of readahead loop

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Message-Id: <20200225214838.30017-13-willy@infradead.org>
Date:   Tue, 25 Feb 2020 13:48:25 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com, ocfs2-devel@....oracle.com,
        linux-xfs@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: [PATCH v8 12/25] mm: Move end_index check out of readahead loop

From: "Matthew Wilcox (Oracle)" <willy@...radead.org>

By reducing nr_to_read, we can eliminate this check from inside the loop.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
Reviewed-by: John Hubbard <jhubbard@...dia.com>
---
 mm/readahead.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index d01531ef9f3c..a37b68f66233 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -167,8 +167,6 @@ void __do_page_cache_readahead(struct address_space *mapping,
 		unsigned long lookahead_size)
 {
 	struct inode *inode = mapping->host;
-	struct page *page;
-	unsigned long end_index;	/* The last page we want to read */
 	LIST_HEAD(page_pool);
 	loff_t isize = i_size_read(inode);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
@@ -178,22 +176,29 @@ void __do_page_cache_readahead(struct address_space *mapping,
 		._index = index,
 	};
 	unsigned long i;
+	pgoff_t end_index;	/* The last page we want to read */
 
 	if (isize == 0)
 		return;
 
-	end_index = ((isize - 1) >> PAGE_SHIFT);
+	end_index = (isize - 1) >> PAGE_SHIFT;
+	if (index > end_index)
+		return;
+	/* Avoid wrapping to the beginning of the file */
+	if (index + nr_to_read < index)
+		nr_to_read = ULONG_MAX - index + 1;
+	/* Don't read past the page containing the last byte of the file */
+	if (index + nr_to_read >= end_index)
+		nr_to_read = end_index - index + 1;
 
 	/*
 	 * Preallocate as many pages as we will need.
 	 */
 	for (i = 0; i < nr_to_read; i++) {
-		if (index + i > end_index)
-			break;
+		struct page *page = xa_load(&mapping->i_pages, index + i);
 
 		BUG_ON(index + i != rac._index + rac._nr_pages);
 
-		page = xa_load(&mapping->i_pages, index + i);
 		if (page && !xa_is_value(page)) {
 			/*
 			 * Page already present?  Kick off the current batch of
-- 
2.25.0