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

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Message-ID: <c8265d22-5cbb-4211-b91d-87965b8505e2@redhat.com>
Date: Wed, 21 May 2025 13:57:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.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 21.05.25 13:16, Zi Yan wrote:
> 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.

Like existing set_pfnblock_flags_mask() I guess, with the try_cmpxchg() 
loop.

> 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.

We'd still have to do the try_cmpxchg() when dealing with multiple bits, 
right?

For single bits, we could just use set_bit() etc.

-- 
Cheers,

David / dhildenb