linux-kernel - Re: [lib/stackdepot] 1cd8ce52c5: BUG:unable_to_handle_page_fault_for

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Date:   Fri, 15 Oct 2021 10:27:17 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        0day robot <lkp@...el.com>, Dmitry Vyukov <dvyukov@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Vijayanand Jitta <vjitta@...eaurora.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Oliver Glitta <glittao@...il.com>,
        Imran Khan <imran.f.khan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        kasan-dev@...glegroups.com
Subject: Re: [lib/stackdepot] 1cd8ce52c5:
 BUG:unable_to_handle_page_fault_for_address

On 10/14/21 12:16, Mike Rapoport wrote:
> On Thu, Oct 14, 2021 at 11:33:03AM +0200, Vlastimil Babka wrote:
>> On 10/14/21 10:54, kernel test robot wrote:
>> 
>> In my local testing of the patch, when stackdepot was initialized through
>> page owner init, it was using kvmalloc() so slab_is_available() was true.
>> Looks like the exact order of slab vs page_owner alloc is non-deterministic,
>> could be arch-dependent or just random ordering of init calls. A wrong order
>> will exploit the apparent fact that slab_is_available() is not a good
>> indicator of using memblock vs page allocator, and we would need a better one.
>> Thoughts?
> 
> The order of slab vs page_owner is deterministic, but it is different for
> FLATMEM and SPARSEMEM. And page_ext_init_flatmem_late() that initializes
> page_ext for FLATMEM is called exactly between buddy and slab setup:

Oh, so it was due to FLATMEM, thanks for figuring that out!

> static void __init mm_init(void)
> {
> 	...
> 
> 	mem_init();
> 	mem_init_print_info();
> 	/* page_owner must be initialized after buddy is ready */
> 	page_ext_init_flatmem_late();
> 	kmem_cache_init();
> 
> 	...
> }
> 
> I've stared for a while at page_ext init and it seems that the
> page_ext_init_flatmem_late() can be simply dropped because there is anyway
> a call to invoke_init_callbacks() in page_ext_init() that is called much
> later in the boot process.

Yeah, but page_ext_init() only does something for SPARSEMEM, and is empty on
FLATMEM. Otherwise it would be duplicating all the work. So I'll just move
page_ext_init_flatmem_late() below kmem_cache_init() in mm_init(). Thanks
again!