linux-kernel - Re: [patch 2/5] Reinstate ZERO_PAGE optimization in get_user

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Date:	Mon, 23 Jun 2008 10:27:03 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeff Chua <jeff.chua.linux@...il.com>
cc:	Hugh Dickins <hugh@...itas.com>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	Justin Forbes <jmforbes@...uxtx.org>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>,
	Randy Dunlap <rdunlap@...otime.net>,
	Dave Jones <davej@...hat.com>,
	Chuck Wolber <chuckw@...ntumlinux.com>,
	Chris Wedgwood <reviews@...cw.f00f.org>,
	Michael Krufky <mkrufky@...uxtv.org>,
	Chuck Ebbert <cebbert@...hat.com>,
	Domenico Andreoli <cavokz@...il.com>, Willy Tarreau <w@....eu>,
	Rodrigo Rubira Branco <rbranco@...checkpoint.com>,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Oleg Nesterov <oleg@...sign.ru>, Nick Piggin <npiggin@...e.de>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, Roland McGrath <roland@...hat.com>
Subject: Re: [patch 2/5] Reinstate ZERO_PAGE optimization in get_user_pages()
 and fix XIP



On Tue, 24 Jun 2008, Jeff Chua wrote:
> On Tue, Jun 24, 2008 at 12:39 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Jeff, does this still work with vmware?
> 
> 
> No, this breaks vmware. Does this trace help?

Not really. I have no idea what vmware does, so any traces from vmware are 
pretty useless.

On the other hand, if you add a trace to the "use_zero_page()" function to 
print out the vm_flags and other details, that probably would help.

That said, since the previous patch _did_ work, I bet that one that does 
both VM_LOCKED and VM_SHARED works too. There was a reason I wanted to do 
that VM_SHARED test. I think the VM_SHARED test is sane, unlike the 
VM_LOCKED test (that is a fairly dubious hack for mlock).

So here's the final version. I bet it works.

		Linus
---
 mm/memory.c |   23 +++++++++++++++++++++--
 1 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 9aefaae..423e0e7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1045,6 +1045,26 @@ no_page_table:
 	return page;
 }
 
+/* Can we do the FOLL_ANON optimization? */
+static inline int use_zero_page(struct vm_area_struct *vma)
+{
+	/*
+	 * We don't want to optimize FOLL_ANON for make_pages_present()
+	 * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
+	 * we want to get the page from the page tables to make sure
+	 * that we serialize and update with any other user of that
+	 * mapping.
+	 */
+	if (vma->vm_flags & (VM_LOCKED | VM_SHARED))
+		return 0;
+	/*
+	 * And if we have a fault or a nopfn routine, it's not an
+	 * anonymous region.
+	 */
+	return !vma->vm_ops ||
+		(!vma->vm_ops->fault && !vma->vm_ops->nopfn);
+}
+
 int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		unsigned long start, int len, int write, int force,
 		struct page **pages, struct vm_area_struct **vmas)
@@ -1119,8 +1139,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		foll_flags = FOLL_TOUCH;
 		if (pages)
 			foll_flags |= FOLL_GET;
-		if (!write && !(vma->vm_flags & VM_LOCKED) &&
-		    (!vma->vm_ops || !vma->vm_ops->fault))
+		if (!write && use_zero_page(vma))
 			foll_flags |= FOLL_ANON;
 
 		do {
--
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