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Date:   Thu, 23 Jul 2020 20:08:46 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal Hocko <mhocko@suse.com>, stable@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Minchan Kim <minchan@kernel.org>,
        Huang Ying <ying.huang@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and
 don't read garbage memmaps
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On Tue, 23 Jun 2020 17:30:18 +0800 Wei Yang <richard.weiyang@linux.alibaba.com> wrote:

> On Tue, Jun 23, 2020 at 09:55:43AM +0200, David Hildenbrand wrote:
> >On 23.06.20 09:39, David Hildenbrand wrote:
> >>> Hmm.. I thought this is the behavior for early section, while it looks current
> >>> code doesn't work like this:
> >>>
> >>>        if (section_is_early && memmap)
> >>>                free_map_bootmem(memmap);
> >>>        else
> >>> 	       depopulate_section_memmap(pfn, nr_pages, altmap);
> >>>
> >>> section_is_early is always "true" for early section, while memmap is not-NULL
> >>> only when sub-section map is empty.
> >>>
> >>> If my understanding is correct, when we remove a sub-section in early section,
> >>> the code would call depopulate_section_memmap(), which in turn free related
> >>> memmap. By removing the memmap, the return value from pfn_to_online_page() is
> >>> not a valid one.
> >> 
> >> I think you're right, and pfn_valid() would also return true, as it is
> >> an early section. This looks broken.
> >> 
> >>>
> >>> Maybe we want to write the code like this:
> >>>
> >>>        if (section_is_early)
> >>>                if (memmap)
> >>>                        free_map_bootmem(memmap);
> >>>        else
> >>> 	       depopulate_section_memmap(pfn, nr_pages, altmap);
> >>>
> >> 
> >> I guess that should be the way to go
> >> 
> >> @Dan, I think what Wei proposes here is correct, right? Or how does it
> >> work in the VMEMMAP case with early sections?
> >> 
> >
> >Especially, if you would re-hot-add, section_activate() would assume
> >there is a memmap, it must not be removed.
> >
> 
> You are right here. I didn't notice it.
> 
> >@Wei, can you send a patch?
> >
> 
> Sure, let me prepare for it.

Still awaiting this, and the v3 patch was identical to this v2 patch.

It's tagged for -stable, so there's some urgency.  Should we just go
ahead with the decently-tested v2?