linux-kernel - Re: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack

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Message-ID: <alpine.LFD.2.00.1005091245000.3711@i5.linux-foundation.org>
Date:	Sun, 9 May 2010 12:56:49 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and
 rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary
 stack



On Sun, 9 May 2010, Mel Gorman wrote:
> 
> It turns out not to be easy to the preallocating of PUDs, PMDs and PTEs
> move_page_tables() needs.  To avoid overallocating, it has to follow the same
> logic as move_page_tables duplicating some code in the process. The ugliest
> aspect of all is passing those pre-allocated pages back into move_page_tables
> where they need to be passed down to such functions as __pte_alloc. It turns
> extremely messy.

Umm. What?

That's crazy talk. I'm not talking about preallocating stuff in order to 
pass it in to move_page_tables(). I'm talking about just _creating_ the 
dang page tables early - preallocating them IN THE PROCESS VM SPACE.

IOW, a patch like this (this is a pseudo-patch, totally untested, won't 
compile, yadda yadda - you need to actually make the people who call 
"move_page_tables()" call that prepare function first etc etc)

Yeah, if we care about holes in the page tables, we can certainly copy 
more of the move_page_tables() logic, but it certainly doesn't matter for 
execve(). This just makes sure that the destination page tables exist 
first.

		Linus

---
 mm/mremap.c |   22 +++++++++++++++++++++-
 1 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index cde56ee..c14505c 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -128,6 +128,26 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 
 #define LATENCY_LIMIT	(64 * PAGE_SIZE)
 
+/*
+ * Preallocate the page tables, so that we can do the actual move
+ * without any allocations, and thus no error handling etc.
+ */
+int prepare_move_page_tables(struct vm_area_struct *vma,
+	unsigned long old_addr, struct vm_area_struct *new_vma,
+	unsigned long new_addr, unsigned long len)
+{
+	unsigned long end_addr = new_addr + len;
+
+	while (new_addr < end_addr) {
+		pmd_t *new_pmd;
+		new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
+		if (!new_pmd)
+			return -ENOMEM;
+		new_addr = (new_addr + PMD_SIZE) & PMD_MASK;
+	}
+	return 0;
+}
+
 unsigned long move_page_tables(struct vm_area_struct *vma,
 		unsigned long old_addr, struct vm_area_struct *new_vma,
 		unsigned long new_addr, unsigned long len)
@@ -147,7 +167,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 		old_pmd = get_old_pmd(vma->vm_mm, old_addr);
 		if (!old_pmd)
 			continue;
-		new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
+		new_pmd = get_old_pmd(vma->vm_mm, new_addr);
 		if (!new_pmd)
 			break;
 		next = (new_addr + PMD_SIZE) & PMD_MASK;
--
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