linux-kernel - [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

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Date:   Sat, 23 Dec 2017 12:16:08 +0800
From:   "十刀" <shidao.ytt@...baba-inc.com>
To:     mgorman@...hsingularity.net, green@...uxhacker.ru
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        "杨勇(智彻)" <zhiche.yy@...baba-inc.com>,
        "夷则(Caspar)" <jinli.zjl@...baba-inc.com>,
        "十刀" <shidao.ytt@...baba-inc.com>
Subject: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

From: "shidao.ytt" <shidao.ytt@...baba-inc.com>

in commit 441c228f817f7 ("mm: fadvise: document the
fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
explained why partial pages should be preserved instead of discarded
when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
end_index was unexpectedly wrong, the code behavior didn't match to the
statement in comments; Luckily in another commit 18aba41cbf
("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
Oleg Drokin fixed this behavior

Here I come up with a new idea that actually we can still discard the
last parital page iff the page-unaligned endbyte is also the end of
file, since no one else will use the rest of the page and it should be
safe enough to discard.

Signed-off-by: shidao.ytt <shidao.ytt@...baba-inc.com>
Signed-off-by: Caspar Zhang <jinli.zjl@...baba-inc.com>
---
 mm/fadvise.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index ec70d6e..f74b21e 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -127,7 +127,8 @@
 		 */
 		start_index = (offset+(PAGE_SIZE-1)) >> PAGE_SHIFT;
 		end_index = (endbyte >> PAGE_SHIFT);
-		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK) {
+		if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK &&
+				endbyte != inode->i_size - 1) {
 			/* First page is tricky as 0 - 1 = -1, but pgoff_t
 			 * is unsigned, so the end_index >= start_index
 			 * check below would be true and we'll discard the whole
-- 
1.8.3.1