Re: [PATCH v5 00/12] MADV_FREE support

On Thu, Jan 28, 2016 at 08:16:25AM +0100, Michael Kerrisk (man-pages) wrote:

Hello Minchan,

On 11/30/2015 07:39 AM, Minchan Kim wrote:

quoted

In v4, Andrew wanted to settle in old basic MADV_FREE and introduces
new stuffs(ie, lazyfree LRU, swapless support and lazyfreeness) later
so this version doesn't include them.

I have been tested it on mmotm-2015-11-25-17-08 with additional
patch[1] from Kirill to prevent BUG_ON which he didn't send to
linux-mm yet as formal patch. With it, I couldn't find any
problem so far.

Note that this version is based on THP refcount redesign so
I needed some modification on MADV_FREE because split_huge_pmd
doesn't split a THP page any more and pmd_trans_huge(pmd) is not
enough to guarantee the page is not THP page.
As well, for MAVD_FREE lazy-split, THP split should respect
pmd's dirtiness rather than marking ptes of all subpages dirty
unconditionally. Please, review last patch in this patchset.

Now that MADV_FREE has been merged, would you be willing to write
patch to the madvise(2) man page that describes the semantics, 
noes limitations and restrictions, and (ideally) has some sentences
describing use cases?

Hello Michael,

Could you review this patch?

Thanks.

From 203372f901f574e991215fdff6907608ba53f932 Mon Sep 17 00:00:00 2001

From: Minchan Kim <redacted>
Date: Fri, 5 Feb 2016 11:09:54 +0900
Subject: [PATCH] madvise.2: Add MADV_FREE

Document the MADV_FREE flags added to madvise() in Linux 4.5

Signed-off-by: Minchan Kim <redacted>
---
 man2/madvise.2 | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/man2/madvise.2 b/man2/madvise.2
index c1df67c..4704304 100644
--- a/man2/madvise.2
+++ b/man2/madvise.2

@@ -143,6 +143,25 @@ flag are special memory areas that are not managed
 by the virtual memory subsystem.
 Such pages are typically created by device drivers that
 map the pages into user space.)
+.TP
+.B MADV_FREE " (since Linux 4.5)"
+Application is finished with the given range, so kernel can free
+resources associated with it but the freeing could be delayed until
+memory pressure happens or canceld by write operation by user.
+
+After a successful MADV_FREE operation, user shouldn't expect kernel
+keeps stale data on the page. However, subsequent write of pages
+in the range will succeed and then kernel cannot free those dirtied pages
+so user can always see just written data. If there was no subsequent
+write, kernel can free those clean pages any time. In such case,
+user can see zero-fill-on-demand pages.
+
+Note that, it works only with private anonymous pages (see
+.BR mmap (2)).
+On swapless system, freeing pages in given range happens instantly
+regardless of memory pressure.
+
+
 .\"
 .\" ======================================================================
 .\"

-- 
1.9.1