linux-kernel - Re: [RFC] a question about mlockall() and mprotect()

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Message-ID: <20170926110012.jiw6plglsyksj5mc@dhcp22.suse.cz>
Date:   Tue, 26 Sep 2017 13:00:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Xishi Qiu <qiuxishi@...wei.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        zhong jiang <zhongjiang@...wei.com>,
        yeyunfeng <yeyunfeng@...wei.com>, wanghaitao12@...wei.com,
        "Zhoukang (A)" <zhoukang7@...wei.com>
Subject: Re: [RFC] a question about mlockall() and mprotect()

On Tue 26-09-17 11:45:16, Vlastimil Babka wrote:
> On 09/26/2017 11:22 AM, Xishi Qiu wrote:
> > On 2017/9/26 17:13, Xishi Qiu wrote:
> >>> This is still very fuzzy. What are you actually trying to achieve?
> >>
> >> I don't expect page fault any more after mlock.
> >>
> > 
> > Our apps is some thing like RT, and page-fault maybe cause a lot of time,
> > e.g. lock, mem reclaim ..., so I use mlock and don't want page fault
> > any more.
> 
> Why does your app then have restricted mprotect when calling mlockall()
> and only later adjusts the mprotect?

Ahh, OK I see what is goging on. So you have PROT_NONE vma at the time
mlockall and then later mprotect it something else and want to fault all
that memory at the mprotect time?

So basically to do
---
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 6d3e2f082290..b665b5d1c544 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -369,7 +369,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
 	 * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
 	 * fault on access.
 	 */
-	if ((oldflags & (VM_WRITE | VM_SHARED | VM_LOCKED)) == VM_LOCKED &&
+	if ((oldflags & (VM_WRITE | VM_LOCKED)) == VM_LOCKED &&
 			(newflags & VM_WRITE)) {
 		populate_vma_page_range(vma, start, end, NULL);
 	}

-- 
Michal Hocko
SUSE Labs