linux-ext4 - [PATCH v2] ext4: fix prefetchw of NULL page

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Date:   Sun, 31 Mar 2019 16:11:34 +0800
From:   Liu Xiang <liu.xiang6@....com.cn>
To:     linux-ext4@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, liuxiang_1999@....com,
        Liu Xiang <liu.xiang6@....com.cn>
Subject: [PATCH v2] ext4: fix prefetchw of NULL page

In ext4_mpage_readpages(), if the parameter pages is not NULL, another
parameter page is NULL. At the first time prefetchw(&page->flags)
works on NULL. From second time, prefetchw(&page->flags) always works on
the last consumed page. This might do little improvment for handling
current page. So prefetchw() should be called while the page pointer
has just been updated.

Signed-off-by: Liu Xiang <liu.xiang6@....com.cn>
---

Changes in v2:
 remove prefetchw() when pages is NULL and change comment

 fs/ext4/readpage.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index 6aa282e..f138c47 100644
--- a/fs/ext4/readpage.c
+++ b/fs/ext4/readpage.c
@@ -126,9 +126,10 @@ int ext4_mpage_readpages(struct address_space *mapping,
 		int fully_mapped = 1;
 		unsigned first_hole = blocks_per_page;
 
-		prefetchw(&page->flags);
 		if (pages) {
 			page = lru_to_page(pages);
+
+			prefetchw(&page->flags);
 			list_del(&page->lru);
 			if (add_to_page_cache_lru(page, mapping, page->index,
 				  readahead_gfp_mask(mapping)))
-- 
1.9.1