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Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section
To:     Wei Yang <richard.weiyang@linux.alibaba.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Linux MM <linux-mm@kvack.org>, Baoquan He <bhe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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From:   David Hildenbrand <david@redhat.com>
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Date:   Tue, 30 Jun 2020 09:16:09 +0200
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On 30.06.20 00:58, Wei Yang wrote:
> On Mon, Jun 29, 2020 at 03:13:25PM -0700, Dan Williams wrote:
>> On Mon, Jun 29, 2020 at 1:34 AM Wei Yang
>> <richard.weiyang@linux.alibaba.com> wrote:
>>>
>>> On Thu, Jun 25, 2020 at 12:46:43PM -0700, Dan Williams wrote:
>>>> On Wed, Jun 24, 2020 at 10:53 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>>> Am 25.06.2020 um 01:47 schrieb Dan Williams <dan.j.williams@intel.com>:
>>>>>>
>>>>>> ﻿On Wed, Jun 24, 2020 at 3:44 PM Wei Yang
>>>>>> <richard.weiyang@linux.alibaba.com> wrote:
>>>>>> [..]
>>>>>>>> So, you are right that there is a mismatch here, but I think the
>>>>>>>> comprehensive fix is to allow early sections to be partially
>>>>>>>> depopulated/repopulated rather than have section_activate() and
>>>>>>>> section_deacticate() special case early sections. The special casing
>>>>>>>> is problematic in retrospect as section_deactivate() can't be
>>>>>>>> maintained without understand special rules in section_activate().
>>>>>>>
>>>>>>> Hmm... This means we need to adjust pfn_valid() too, which always return true
>>>>>>> for early sections.
>>>>>>
>>>>>> Right, rather than carry workarounds in 3 locations, and the bug that
>>>>>> has resulted from then getting out of sync, just teach early section
>>>>>> mapping to allow for the subsection populate/depopulate.
>>>>>>
>>>>>
>>>>> I prefer the easy fix first - IOW what we Here here. Especially, pfn_to_online_page() will need changes as well.
>>>>
>>>> Agree, yes, let's do the simple fix first for 5.8 and the special-case
>>>> elimination work later.
>>>
>>> Dan,
>>>
>>> A quick test shows this is not a simple task.
>>
>> Thanks for taking a look...
>>
>>> First, early sections don't set subsection bitmap, which is necessary for the
>>> hot-add/remove.
>>>
>>> To properly set subsection bitmap, we need to know how many subsections in
>>> early section. While current code doesn't has a alignment requirement for
>>> last early section. We mark the whole last early section as present.
>>
>> I was thinking that the subsection map does not need to be accurate on
>> initial setup, it only needs to be accurate after the first removal.
>> However, that would result in new special casing that somewhat defeats
>> the purpose. The hardest part is potentially breaking up a PMD mapping
>> of the page array into a series of PTE mappings without disturbing
>> in-flight pfn_to_page() users.
>>
>>> I don't find a way to enable this.
>>
>> While I don't like that this bug crept into the mismatched special
>> casing of early sections, I'm now coming around to the same opinion.
>> I.e. that making the memmap for early sections permanent is a simpler
>> mechanism to maintain.
> 
> I think so ...
> 

Yes, and I think having to replace quite some pfn_valid_within() - nops
- by pfn_valid() just to handle one corner case might not be worth it.
At least for now.

-- 
Thanks,

David / dhildenb