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SubjectRe: thp and memory barrier assumptions
On Thu, Jul 26, 2012 at 10:33:25PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-07-26 at 22:31 +0200, Peter Zijlstra wrote:
> > __do_huge_pmd_anonymous_page() contains:
> >
> > /*
> > * The spinlocking to take the lru_lock inside
> > * page_add_new_anon_rmap() acts as a full memory
> > * barrier to be sure clear_huge_page writes become
> > * visible after the set_pmd_at() write.
> > */
> > page_add_new_anon_rmap(page, vma, haddr);
> >
> >
> > page_add_new_anon_rmap() doesn't look to actually do a LOCK+UNLOCK
> > except for unevictable pages.
> >
> > But even if it did do an unconditional LOCK+UNLOCK that doesn't make a
> > full memory barrier, see Documentation/memory-barriers.txt.
> >
> > In particular:
> >
> > *A = a;
> > LOCK
> > UNLOCK
> > *B = b;
> >
> > may occur as:
> >
> > LOCK, STORE *B, STORE *A, UNLOCK
> >
>

I fixed that last year (I think Mel pointed out the bug) but I've been
so busy with other things I forgot to push that theoretical fix from
aa.git to -mm. As soon as autonuma is merged, I'll return to focus on
pushing the other pending patches in my queue that are being starved.

http://git.kernel.org/?p=linux/kernel/git/andrea/aa.git;a=commitdiff;h=d598a3f7ae4ca9d2c2a8653fbe790aab9b1a3141

Can you review it? If ok I'll submit it so it won't starve no
more. Also note the other bugfix that was in fair.c I think is only
needed with AutoNUMA applied this is why I didn't submit it
separately.

This can't affect x86 where even a locked bitop is the equivalent of a
full memory barrier.

> Also, what is that barrier() in handle_mm_fault() doing? And why doesn't
> it have a comment explaining that?

I added the docs below:

=====
From ad51771a2c3fa697fa0267edda23b48d0b85f023 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <aarcange@redhat.com>
Date: Fri, 3 Aug 2012 21:10:44 +0200
Subject: [PATCH] thp: document barrier() in wrprotect THP fault path

Inline doc.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
mm/memory.c | 6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 420a449..9ec5bba 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3522,6 +3522,12 @@ retry:
pmd_t orig_pmd = *pmd;
int ret;

+ /*
+ * flush orig_pmd on the stack to avoid invalidating
+ * the pmd_trans_huge(orig_pmd) check and to allow
+ * do_huge_pmd_wp_page to run a reliable
+ * pmd_same(*pmd, orig_pmd).
+ */
barrier();
if (pmd_trans_huge(orig_pmd)) {
if (flags & FAULT_FLAG_WRITE &&

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