linux-kernel - [PATCH 1/2] mm: fadvise: Document the fadvise(FADV

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed,  3 Dec 2014 00:42:46 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>
Subject: [PATCH 1/2] mm: fadvise: Document the fadvise(FADV_DONTNEED) behaviour for partial pages

A random seek IO benchmark appeared to regress because of a change to
readahead but the real problem was the benchmark. To ensure the IO request
accesssed disk, it used fadvise(FADV_DONTNEED) on a block boundary (512K)
but the hint is ignored by the kernel. This is correct but not necessarily
obvious behaviour.  As much as I dislike comment patches, the explanation
for this behaviour predates current git history. Clarify why it behaves
like this in case someone "fixes" fadvise or readahead for the wrong reasons.

Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/fadvise.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index 3bcfd81d..c908c72 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -117,7 +117,11 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
 			__filemap_fdatawrite_range(mapping, offset, endbyte,
 						   WB_SYNC_NONE);
 
-		/* First and last FULL page! */
+		/*
+		 * First and last FULL page! Partial pages are deliberately
+		 * preserved on the expectation that it is better to preserve
+		 * needed memory than to discard unneeded memory.
+		 */
 		start_index = (offset+(PAGE_CACHE_SIZE-1)) >> PAGE_CACHE_SHIFT;
 		end_index = (endbyte >> PAGE_CACHE_SHIFT);
 
-- 
2.1.2

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/