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

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Message-ID: <e9b85836-b4d9-4678-a59b-dbaf916fa1c5@redhat.com>
Date: Wed, 21 May 2025 14:11:44 +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 14:00, Zi Yan wrote:
> On 21 May 2025, at 7:57, David Hildenbrand wrote:
> 
>> 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.
> 
> Are you saying I duplicate the code in set_pfnblock_flags_mask() to implement
> set_pfnblock_flags()? Or just replace set_pfnblock_flags_mask() entirely?

The latter as possible.

> 
>>
>>> 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.
> 
> Mel moved from set_bit() to try_cmpxchg() a word for performance reason. I am
> not sure we want to move back.

In e58469bafd05 we moved from multiple set_bit etc to a cmpxchange.

-       for (; start_bitidx <= end_bitidx; start_bitidx++, value <<= 1)
-               if (flags & value)
-                       __set_bit(bitidx + start_bitidx, bitmap);
-               else
-                       __clear_bit(bitidx + start_bitidx, bitmap);


However, when only setting/clearing a single bit (e.g., isolated), 
set_bit etc should be much cheaper.

For multiple bits, the existing try_cmpxchg should be kept IMHO.

-- 
Cheers,

David / dhildenb