linux-kernel - Re: [PATCH 1/2] mm: remove pfn_valid_within() and CONFIG_HOLES_IN

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Message-ID: <4367d5e4-4f03-6d99-f19b-9d32b71f227d@redhat.com>
Date:   Tue, 13 Jul 2021 12:24:41 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm: remove pfn_valid_within() and
 CONFIG_HOLES_IN_ZONE

On 13.07.21 12:22, Mike Rapoport wrote:
> On Tue, Jul 13, 2021 at 11:51:46AM +0200, David Hildenbrand wrote:
>> On 13.07.21 10:00, Mike Rapoport wrote:
>>> From: Mike Rapoport <rppt@...ux.ibm.com>
>>>
>>> After recent changes in freeing of the unused parts of the memory map and
>>> rework of pfn_valid() in arm and arm64 there are no architectures that can
>>> have holes in the memory map within a pageblock and so nothing can enable
>>> CONFIG_HOLES_IN_ZONE which guards non trivial implementation of
>>> pfn_valid_within().
>>>
>>> With that, pfn_valid_within() is always hardwired to 1 and can be
>>> completely removed.
>>>
>>> Remove calls to pfn_valid_within() and CONFIG_HOLES_IN_ZONE.
>>>
>>> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
>>
>> There is currently the discussion to increase MAX_ORDER, for example, to
>> cover 1GiB instead of 4MiB on x86-64. This would mean that we could
>> suddenly, again, have holes insides MAX_ORDER - 1 pages.
>>
>> So I assume if we ever go down that path, we'll need something like this
>> again.
> 
> It depends whether pageblock_order will be also increased. PFN walkers rely
> on continuity of pageblocks rather than MAX_ORDER chunks, so if
> pageblock_order won't change, there won't be need to check for pfn_valid()
> inside a pageblock.

I'm pushing for letting pageblocks stay untouched, so good to know!

(we still have this crazy special case of pageblocks > MAX_ORDER - 1 
right now, which I think we should just eliminate)

-- 
Thanks,

David / dhildenb