Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751445AbaLCAnW (ORCPT ); Tue, 2 Dec 2014 19:43:22 -0500 Received: from cantor2.suse.de ([195.135.220.15]:47130 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750862AbaLCAmu (ORCPT ); Tue, 2 Dec 2014 19:42:50 -0500 From: Mel Gorman To: Andrew Morton , Michael Kerrisk Cc: Linux-MM , LKML , Mel Gorman Subject: [PATCH 1/2] mm: fadvise: Document the fadvise(FADV_DONTNEED) behaviour for partial pages Date: Wed, 3 Dec 2014 00:42:46 +0000 Message-Id: <1417567367-9298-2-git-send-email-mgorman@suse.de> X-Mailer: git-send-email 2.1.2 In-Reply-To: <1417567367-9298-1-git-send-email-mgorman@suse.de> References: <1417567367-9298-1-git-send-email-mgorman@suse.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/