linux-kernel - Re: [PATCH v1 4/5] mm/page_alloc: place pages to tail in __free_pages

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Date:   Fri, 2 Oct 2020 17:10:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Dave Hansen <dave.hansen@...el.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Mike Rapoport <rppt@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH v1 4/5] mm/page_alloc: place pages to tail in
 __free_pages_core()

On 02.10.20 15:41, Michal Hocko wrote:
> On Mon 28-09-20 20:21:09, David Hildenbrand wrote:
>> __free_pages_core() is used when exposing fresh memory to the buddy
>> during system boot and when onlining memory in generic_online_page().
>>
>> generic_online_page() is used in two cases:
>>
>> 1. Direct memory onlining in online_pages().
>> 2. Deferred memory onlining in memory-ballooning-like mechanisms (HyperV
>>    balloon and virtio-mem), when parts of a section are kept
>>    fake-offline to be fake-onlined later on.
>>
>> In 1, we already place pages to the tail of the freelist. Pages will be
>> freed to MIGRATE_ISOLATE lists first and moved to the tail of the freelists
>> via undo_isolate_page_range().
>>
>> In 2, we currently don't implement a proper rule. In case of virtio-mem,
>> where we currently always online MAX_ORDER - 1 pages, the pages will be
>> placed to the HEAD of the freelist - undesireable. While the hyper-v
>> balloon calls generic_online_page() with single pages, usually it will
>> call it on successive single pages in a larger block.
>>
>> The pages are fresh, so place them to the tail of the freelists and avoid
>> the PCP. In __free_pages_core(), remove the now superflouos call to
>> set_page_refcounted() and add a comment regarding page initialization and
>> the refcount.
>>
>> Note: In 2. we currently don't shuffle. If ever relevant (page shuffling
>> is usually of limited use in virtualized environments), we might want to
>> shuffle after a sequence of generic_online_page() calls in the
>> relevant callers.
> 
> It took some time to get through all the freeing paths with subtle
> differences but this looks reasonable. You are mentioning that this
> influences a boot time free memory ordering as well but only very
> briefly. I do not expect this to make a huge difference but who knows.
> It makes some sense to add pages in the order they show up in the
> physical address ordering.

I think boot memory is mostly exposed in the physical address ordering.
In that case, higher addresses will now be used less likely immediately
after this patch. I also don't think it's an issue - if we still detect
it's an issue it's fairly easy to change again.

Thanks!

-- 
Thanks,

David / dhildenb