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

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Date:   Fri, 26 Jun 2020 06:59:11 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Linux MM <linux-mm@...ck.org>, Baoquan He <bhe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section



> Am 26.06.2020 um 00:40 schrieb Wei Yang <richard.weiyang@...ux.alibaba.com>:
> 
> On Thu, Jun 25, 2020 at 07:53:37AM +0200, David Hildenbrand wrote:
>> 
>> 
>>>> Am 25.06.2020 um 01:47 schrieb Dan Williams <dan.j.williams@...el.com>:
>>> 
>>> On Wed, Jun 24, 2020 at 3:44 PM Wei Yang
>>> <richard.weiyang@...ux.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.
>> 
> 
> Hi, David,
> 
> Which part of pfn_to_online_page() needs to be changed? pfn_valid_within()
> would call pfn_valid() to check the pfn first. This looks enough for me.

Not for all configurations. For some (e.g., x86 iirc) it‘s just a nop.