linux-ext4 - [PATCH v6 04/19] mm: Rearrange readahead loop

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Message-Id: <20200217184613.19668-5-willy@infradead.org>
Date:   Mon, 17 Feb 2020 10:45:45 -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
Subject: [PATCH v6 04/19] mm: Rearrange readahead loop

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

Move the declaration of 'page' to inside the loop and move the 'kick
off a fresh batch' code to the end of the function for easier use in
subsequent patches.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 mm/readahead.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 15329309231f..3eca59c43a45 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -154,7 +154,6 @@ 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);
 	int page_idx;
@@ -175,6 +174,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
 	 * Preallocate as many pages as we will need.
 	 */
 	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
+		struct page *page;
 		pgoff_t page_offset = offset + page_idx;
 
 		if (page_offset > end_index)
@@ -183,14 +183,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
 		page = xa_load(&mapping->i_pages, page_offset);
 		if (page && !xa_is_value(page)) {
 			/*
-			 * Page already present?  Kick off the current batch of
-			 * contiguous pages before continuing with the next
-			 * batch.
+			 * Page already present?  Kick off the current batch
+			 * of contiguous pages before continuing with the
+			 * next batch.  This page may be the one we would
+			 * have intended to mark as Readahead, but we don't
+			 * have a stable reference to this page, and it's
+			 * not worth getting one just for that.
 			 */
-			if (readahead_count(&rac))
-				read_pages(&rac, &page_pool, gfp_mask);
-			rac._nr_pages = 0;
-			continue;
+			goto read;
 		}
 
 		page = __page_cache_alloc(gfp_mask);
@@ -201,6 +201,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
 		if (page_idx == nr_to_read - lookahead_size)
 			SetPageReadahead(page);
 		rac._nr_pages++;
+		continue;
+read:
+		if (readahead_count(&rac))
+			read_pages(&rac, &page_pool, gfp_mask);
+		rac._nr_pages = 0;
 	}
 
 	/*
-- 
2.25.0