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 <aarca...@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 <aarca...@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 && -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
