linux-kernel - Re: [PATCH v4 1/4] mm/page_isolation: make page isolation a standalone bit.

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Message-ID: <CA27CAF4-DD4B-4350-90A6-AE9A808F05C0@nvidia.com>
Date: Wed, 21 May 2025 07:16:53 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Oscar Salvador <osalvador@...e.de>, Johannes Weiner <hannes@...xchg.org>,
 linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 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 v4 1/4] mm/page_isolation: make page isolation a
 standalone bit.

On 19 May 2025, at 12:42, David Hildenbrand wrote:

>>>> +#ifdef CONFIG_MEMORY_ISOLATION
>>>> +	if (flags & PB_migrate_isolate_bit)
>>>> +		return MIGRATE_ISOLATE;
>>>> +#endif
>>>
>>> If you call get_pfnblock_flags_mask() with MIGRATETYPE_MASK, how could you ever get PB_migrate_isolate_bit?
>>
>> MIGRATETYPE_MASK is ((BIT(PB_migratetype_bits) - 1) | PB_migrate_isolate_bit),
>> so it gets PB_migrate_isolate_bit.
>>
>
> Oh ... that's confusing.
>
>>>
>>>
>>> I think what we should do is
>>>
>>> 1) Rename get_pfnblock_flags_mask() to get_pfnblock_flags()
>>>
>>> 2) Remove the mask parameter
>>>
>>> 3) Perform the masking in all callers.
>>
>> get_pfnblock_flags_mask() is also used by get_pageblock_skip() to
>> get PB_migrate_skip. I do not think we want to include PB_migrate_skip
>> in the mask to confuse readers.
>
> The masking will be handled in the caller.
>
> So get_pageblock_skip() would essentially do a
>
> return get_pfnblock_flags() & PB_migrate_skip_bit;
>
> etc.
>
>>
>>>
>>>
>>>
>>> Maybe, we should convert set_pfnblock_flags_mask() to
>>>
>>> void set_clear_pfnblock_flags(struct page *page, unsigned long
>>> 			      set_flags, unsigned long clear_flags);
>>>
>>> And better, splitting it up (or providing helpers)
>>>
>>> set_pfnblock_flags(struct page *page, unsigned long flags);
>>> clear_pfnblock_flags(struct page *page, unsigned long flags);
>>>
>>>
>>> This implies some more code cleanups first that make the code easier to extend.
>>>
>>
>> The same due to PB_migrate_skip.
>>
>> Based on your suggestion, we could make {set,get}_pfnblock_flags_mask()
>> internal APIs by prepending "__". They are only used by the new
>> {get, set, clear}_pfnblock_flags() and {get, set, clear}_pageblock_{skip, isolate}().
>> Then use {get, set, clear}_pfnblock_flags() for all migratetype operations.
>>
>> WDYT?
>
> In general, lgtm. I just hope we can avoid the "_mask" part and just handle it in these functions directly?

After implementing {get, set, clear}_pfnblock_flags(), I find that
get_pfnblock_flags() is easy like you wrote above, but set and clear are not,
since migratetype and skip/isolate bits are in the same word, meaning
I will need to first read them out, change the field, then write them back.
But it will cause inconsistency if there is a parallel writer to the same
word. So for set and clear, mask is required.

I can try to implement {get, set, clear}_pfnblock_bits(page,pfn, bits) to
only handle standalone bits by using the given @bits as the mask and
{set,get}_pageblock_migratetype() still use the mask.

WDYT?

--
Best Regards,
Yan, Zi