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

[Zi Yan]

On 8 May 2025, at 16:53, Zi Yan wrote:

> 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.

Hmm, hit another roadblock. In online_pages() from mm/memory_hotplug.c,
move_pfn_range_to_zone(MIGRATE_ISOLATE) calls memmap_init_range(),
which uses set_pageblock_migratetype(MIGRATE_ISOLATE).

I could use set_pageblock_isolate() in memmap_init_range(), but
that requires move set_pageblock_isolate() out of mm/page_isolation.c.
The change might be too substantial for a fixup.

I also would like to get some opinion from David on this. So I am
holding this fixup and will send it out as a separate patch when
I get more information.


--
Best Regards,
Yan, Zi