linux-kernel - Re: mm/percpu.c: use smarter memory allocation for struct pcpu_alloc

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Message-ID: <nycvar.YSQ.7.76.1711271534590.5925@knanqh.ubzr>
Date:   Mon, 27 Nov 2017 15:51:04 -0500 (EST)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Tejun Heo <tj@...nel.org>
cc:     Guenter Roeck <linux@...ck-us.net>,
        Christoph Lameter <cl@...ux.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Mikael Starvik <starvik@...s.com>,
        Jesper Nilsson <jesper.nilsson@...s.com>,
        linux-cris-kernel@...s.com
Subject: Re: mm/percpu.c: use smarter memory allocation for struct pcpu_alloc_info
 (crisv32 hang)

On Mon, 27 Nov 2017, Tejun Heo wrote:

> Hello,
> 
> On Mon, Nov 27, 2017 at 03:31:52PM -0500, Nicolas Pitre wrote:
> > So IMHO I don't think reverting the commit is the right thing to do. 
> > That commit is clearly not at fault here.
> 
> It's not about the blame.  We just want to avoid breaking boot in a
> way which is difficult to debug.  Once cris is fixed, we can re-apply
> the patch.

In that case I suggest the following instead. No point penalizing 
everyone for a single architecture's fault. And this will serve as a 
visible reminder to the cris people that they need to clean up.

----- >8
Subject: percpu: hack to let the CRIS architecture to boot until they clean up

Commit 438a506180 ("percpu: don't forget to free the temporary struct 
pcpu_alloc_info") uncovered a problem on the CRIS architecture where
the bootmem allocator is initialized with virtual addresses. Given it 
has:

    #define __va(x) ((void *)((unsigned long)(x) | 0x80000000))

then things just work out because the end result is the same whether you
give this a physical or a virtual address.

Untill you call memblock_free_early(__pa(address)) that is, because
values from __pa() don't match with the virtual addresses stuffed in the
bootmem allocator anymore.

Avoid freeing the temporary pcpu_alloc_info memory on that architecture
until they fix things up to let the kernel boot like it did before.

Signed-off-by: Nicolas Pitre <nico@...aro.org>

diff --git a/mm/percpu.c b/mm/percpu.c
index 79e3549cab..50e7fdf840 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -2719,7 +2719,11 @@ void __init setup_per_cpu_areas(void)

 	if (pcpu_setup_first_chunk(ai, fc) < 0)
 		panic("Failed to initialize percpu areas.");
+#ifdef CONFIG_CRIS
+#warning "the CRIS architecture has physical and virtual addresses confused"
+#else
 	pcpu_free_alloc_info(ai);
+#endif
 }

 #endif	/* CONFIG_SMP */