On Fri, Jul 22, 2016 at 11:21 AM, Andy Lutomirski [off-list ref] wrote:

On Fri, Jul 22, 2016 at 3:21 AM, Ingo Molnar [off-list ref] wrote:

quoted

* Andy Lutomirski [off-list ref] wrote:

quoted

On 07/21/2016 09:43 PM, Valdis.Kletnieks@vt.edu wrote:

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On Mon, 11 Jul 2016 13:53:36 -0700, Andy Lutomirski said:

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This avoids pointless races in which another CPU or task might see a
partially populated global pgd entry.  These races should normally
be harmless, but, if another CPU propagates the entry via
vmalloc_fault and then populate_pgd fails (due to memory allocation
failure, for example), this prevents a use-after-free of the pgd
entry.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/mm/pageattr.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

I just bisected a failure to boot down to this patch.  On my Dell Latitude
laptop, it results in the kernel being loaded and then just basically sitting
there dead in the water - as far as I can tell, it dies before the kernel
ever gets going far enough to do any console I/O (even with ignore_loglevel).
Nothing in /sys/fs/pstore either.  I admit not understanding the VM code
at all, so I don't have a clue *why* this causes indigestion...

CPU is an Intel Core i5-3340M in case that matters....

How much memory do you have and what's your config?  My code is obviously
buggy, but I'm wondering why neither I nor the 0day bot caught this.

The attached patch is compile-tested only.  (Even Thunderbird doesn't want
to send non-flowed text right now, sigh.)

--Andy

quoted

From 6589ddf69a1369e1ecb95f0af489d90b980e256e Mon Sep 17 00:00:00 2001
Message-Id: [off-list ref]
From: Andy Lutomirski <luto@kernel.org>
Date: Thu, 21 Jul 2016 22:22:02 -0700
Subject: [PATCH] x86/mm: Fix populate_pgd()

I make an obvious error in populate_pgd() -- it would fail to correctly
populate the page tables when it allocated a new pud page.

JFYI, on allnoconfig it gives:

  arch/x86/mm/pageattr.c:1016:20: error: implicit declaration of function ‘pud_index’ [-Werror=implicit-function-declaration]

As it happens, my fix interacts badly with the steaming pile of crap
that is Linux's support for <4 page table levels.  Can you just revert
the offending patch and I'll redo it differently?

No, don't revert it.  The result doesn't work.  I'll send something.