Re: [PATCH] mm/spase: never partially remove memmap for early section

[Wei Yang <richard.weiyang@linux.alibaba.com>]

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 ...

-- 
Wei Yang
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