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

[Qian Cai <cai@lca.pw>]

> On Nov 27, 2019, at 9:13 AM, Michal Hocko <mhocko@suse.com> 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.

Indeed. Too many times people are only able to reproduce the issues on
old kernels but insist to forward-fix the mainline as well which only bring
unstable there.