Re: [RFC PATCH] mm, page_alloc: avoid page_to_pfn() in move_freepages()

From: Kefeng Wang <wangkefeng.wang@huawei.com>
To: Michal Hocko <mhocko@suse.com>
Cc: <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>,
	"Vlastimil Babka" <vbabka@suse.cz>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [RFC PATCH] mm, page_alloc: avoid page_to_pfn() in move_freepages()
Date: Wed, 27 Nov 2019 22:39:56 +0800	[thread overview]
Message-ID: <b430c512-eb70-6b8e-4f4e-98617e87ee58@huawei.com> (raw)
In-Reply-To: <20191127141340.GA26807@dhcp22.suse.cz>

On 2019/11/27 22:13, Michal Hocko wrote:
> On Wed 27-11-19 21:13:00, Kefeng Wang wrote:
>>
>>
>> On 2019/11/27 19:47, Michal Hocko wrote:
>>> On Wed 27-11-19 18:28:00, Kefeng Wang wrote:
>>>> The start_pfn and end_pfn are already available in move_freepages_block(),
>>>> pfn_valid_within() should validate pfn first before touching the page,
>>>> or we might access an unitialized page with CONFIG_HOLES_IN_ZONE configs.
>>>>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---
>>>>
>>>> Here is an oops in 4.4(arm64 enabled CONFIG_HOLES_IN_ZONE),
>>>
>>> Is this reproducible with the current upstream kernel? There were large
>>> changes in this aread since 4.4
>>
>> Our inner tester found this oops twice, but couldn't be reproduced for now,
>> even in 4.4 kernel, still trying...
>>
>> But the page_to_pfn() shouldn't be used in move_freepages(), right? ; )
> 
> Well, I do agree that going back and forth between page and pfn is ugly.
> So this as a cleanup makes sense to me. But you are trying to fix a bug
> and that bug should be explained. NULL ptr dereference sounds like a
> memmap is not allocated for the particular pfn and this is a bit
> unexpected even with holes, at least on x86, maybe arm64 allows that.
> But the changelog should be clear about all this rather than paper over
> a deeper problem potentially. Please also make sure to involve arm64
> people.

I'm still trying to reproduce it on 4.4 and 5.4, add Catalin, Will Mark,
could you give some advice on it, thanks.

https://lore.kernel.org/linux-mm/54064878-ea85-247a-3382-b96ddf97c667@huawei.com/T/#m87c545730a0a00c45e042937593c59f6552d1246

note:
We backport numa patches into 4.4, so the CONFIG_HOLES_IN_ZONE is enabled.

# CONFIG_NUMA is not set
CONFIG_HOLES_IN_ZONE=y

CONFIG_SPARSEMEM_MANUAL=y
CONFIG_SPARSEMEM=y
CONFIG_HAVE_MEMORY_PRESENT=y
CONFIG_SPARSEMEM_EXTREME=y
CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
CONFIG_SPARSEMEM_ALLOC_MEM_MAP_TOGETHER=y
# CONFIG_SPARSEMEM_VMEMMAP is not set

> 