linux-kernel - Re: [PATCH v3 1/4] mm/page_isolation: make page isolation a standalone bit.

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Message-ID: <10C2C1DD-A6E2-4745-8C2E-D09886F4104E@nvidia.com>
Date: Thu, 08 May 2025 16:53:37 -0400
From: Zi Yan <ziy@...dia.com>
To: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
 Oscar Salvador <osalvador@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Mel Gorman <mgorman@...hsingularity.net>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Brendan Jackman <jackmanb@...gle.com>, Richard Chang <richardycc@...gle.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] mm/page_isolation: make page isolation a
 standalone bit.

On 8 May 2025, at 16:46, Johannes Weiner wrote:

> On Thu, May 08, 2025 at 03:17:05PM -0400, Zi Yan wrote:
>>
>>>>> @@ -426,7 +460,12 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
>>>>>  		     migratetype < MIGRATE_PCPTYPES))
>>>>>  		migratetype = MIGRATE_UNMOVABLE;
>>>>>
>>>>> -	set_pfnblock_flags_mask(page, (unsigned long)migratetype,
>>>>> +#ifdef CONFIG_MEMORY_ISOLATION
>>>>> +	if (migratetype == MIGRATE_ISOLATE)
>>>>> +		set_pageblock_isolate(page);
>>>>
>>>> Are there paths actually doing this after the second patch?
>>>>
>>>> There are many instances that want to *read* the migratetype or
>>>> MIGRATE_ISOLATE, but only isolation code should be manipulating that
>>>> bit through the dedicated set/toggle_pageblock_isolate API.
>>>>
>>>> If there isn't one, it might be good to enforce this with a VM_WARN
>>>> instead.
>>>
>>> I checked all set_pageblock_migratetype() callers and do not see
>>> one using it for pageblock isolation. Let me replace the code
>>> with a VM_WARN and add a comment to tell users to use dedicated
>>> pageblock isolation APIs.
>>>
>>
>> Actually, move_freepages_block_isolate() calls __move_freepages_block()
>> to move free pages to MIGRATE_ISOLATE pageblock and
>> set_pageblock_migratetype() is used inside __move_freepages_block().
>> So the branch has to stay. Will use the suggestion below.
>
> Ah, good catch. But looking at the callers, it's:
>
> move_freepages_block()
> move_freepages_block_isolate()
> try_to_claim_block()
>
> The last one would benefit from having the set_pageblock_migratetype()
> there explicitly, as this is what this function is supposed to do. It
> also should never set the isolation bit.
>
> move_freepages_block_isolate() has two set_pageblock_migratetype()
> calls already. And after the series, it should only manipulate the
> isolate bit, not change the actual migratetype anymore, right?
>
> Maybe it makes the most sense to move it into the three callers?
>
> And then fortify set_pageblock_migratetype() after all.

Sounds good to me. Let me update my fixups.

Hi Andrew,

I will resend new fixups based on Johannes' comment.

--
Best Regards,
Yan, Zi