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From:   Zi Yan <ziy@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene
Date:   Mon, 02 Oct 2023 22:35:10 -0400
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On 2 Oct 2023, at 7:43, David Hildenbrand wrote:

>>>> I can do it after I fix this. That change might or might not help on=
ly if we make
>>>> some redesign on how migratetype is managed. If MIGRATE_ISOLATE does=
 not
>>>> overwrite existing migratetype, the code might not need to split a p=
age and move
>>>> it to MIGRATE_ISOLATE freelist?
>>>
>>> Did someone test how memory offlining plays along with that? (I can t=
ry myself
>>> within the next 1-2 weeks)
>>>
>>> There [mm/memory_hotplug.c:offline_pages] we always cover full MAX_OR=
DER ranges,
>>> though.
>>>
>>> ret =3D start_isolate_page_range(start_pfn, end_pfn,
>>> 			       MIGRATE_MOVABLE,
>>> 			       MEMORY_OFFLINE | REPORT_FAILURE,
>>> 			       GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL);
>>
>> Since a full MAX_ORDER range is passed, no free page split will happen=
=2E
>
> Okay, thanks for verifying that it should not be affected!
>
>>
>>>
>>>>
>>>> The fundamental issue in alloc_contig_range() is that to work at
>>>> pageblock level, a page (>pageblock_order) can have one part is isol=
ated and
>>>> the rest is a different migratetype. {add_to,move_to,del_page_from}_=
free_list()
>>>> now checks first pageblock migratetype, so such a page needs to be r=
emoved
>>>> from its free_list, set MIGRATE_ISOLATE on one of the pageblock, spl=
it, and
>>>> finally put back to multiple free lists. This needs to be done at is=
olation stage
>>>> before free pages are removed from their free lists (the stage after=
 isolation).
>>>
>>> One idea was to always isolate larger chunks, and handle movability c=
hecks/split/etc
>>> at a later stage. Once isolation would be decoupled from the actual/o=
riginal migratetype,
>>> the could have been easier to handle (especially some corner cases I =
had in mind back then).
>>
>> I think it is a good idea. When I coded alloc_contig_range() up, I tri=
ed to
>> accommodate existing set_migratetype_isolate(), which calls has_unmova=
ble_pages().
>> If these two are decoupled, set_migrateype_isolate() can work on MAX_O=
RDER-aligned
>> ranges and has_unmovable_pages() can still work on pageblock-aligned r=
anges.
>> Let me give this a try.
>>
>
> But again, just some thought I had back then, maybe it doesn't help for=
 anything; I found more time to look into the whole thing in more detail.=


Sure. The devil is in the details, but I will only know the details and w=
hat works
after I code it up. :)

>>>
>>>> If MIGRATE_ISOLATE is a separate flag and we are OK with leaving iso=
lated pages
>>>> in their original migratetype and check migratetype before allocatin=
g a page,
>>>> that might help. But that might add extra work (e.g., splitting a pa=
rtially
>>>> isolated free page before allocation) in the really hot code path, w=
hich is not
>>>> desirable.
>>>
>>> With MIGRATE_ISOLATE being a separate flag, one idea was to have not =
a single
>>> separate isolate list, but one per "proper migratetype". But again, j=
ust some random
>>> thoughts I had back then, I never had sufficient time to think it all=
 through.
>>
>> Got it. I will think about it.
>>
>> One question on separate MIGRATE_ISOLATE:
>>
>> the implementation I have in mind is that MIGRATE_ISOLATE will need a =
dedicated flag
>> bit instead of being one of migratetype. But now there are 5 migratety=
pes +
>
> Exactly what I was concerned about back then ...
>
>> MIGRATE_ISOLATE and PB_migratetype_bits is 3, so an extra migratetype_=
bit is needed.
>> But current migratetype implementation is a word-based operation, requ=
iring
>> NR_PAGEBLOCK_BITS to be divisor of BITS_PER_LONG. This means NR_PAGEBL=
OCK_BITS
>> needs to be increased from 4 to 8 to meet the requirement, wasting a l=
ot of space.
>
> ... until I did the math. Let's assume a pageblock is 2 MiB.
>
> 4/(2* 1024 * 1024 * 8) =3D 0,00002384185791016 %
>
> 8/(2* 1024 * 1024 * 8) -> 1 / (2* 1024 * 1024) =3D 0,00004768371582031 =
%
>
> For a 1 TiB machine that means 256 KiB vs. 512 KiB
>
> I concluded that "wasting a lot of space" is not really the right word =
to describe that :)
>
> Just to put it into perspective, the memmap (64/4096) for a 1 TiB machi=
ne is ... 16 GiB.

You are right. I should have done the math. The absolute increase is not =
much.

>> An alternative is to have a separate array for MIGRATE_ISOLATE, which =
requires
>> additional changes. Let me know if you have a better idea. Thanks.
>
> It would probably be cleanest to just use one byte per pageblock. That =
would cleanup the whole machinery eventually as well.

Let me give this a try and see if it cleans things up.


--
Best Regards,
Yan, Zi

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