linux-kernel - Re: [PATCH RFC 0/4] mm: Remember young bit for migration entries

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Date:   Tue, 2 Aug 2022 22:23:49 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Nadav Amit <nadav.amit@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH RFC 0/4] mm: Remember young bit for migration entries

> I don't think we only care about x86_64?  Should other archs have the same
> issue as long as there's the hardware young bit?
> 
> Even without it, it'll affect page reclaim logic too, and that's also not
> x86 only.

Okay, reading the cover letter and looking at the code my understanding
was that x86-64 is the real focus.

>>
>>>
>>> Besides I actually have a question on the anon exclusive bit in the swap
>>> pte: since we have that anyway, why we need a specific migration type for
>>> anon exclusive pages?  Can it be simply read migration entries with anon
>>> exclusive bit set?
>>
>> Not before all arch support pte_swp_mkexclusive/pte_swp_exclusive/.
>>
>> As pte_swp_mkexclusive/pte_swp_exclusive/ only applies to actual swap
>> PTEs, you could even reuse that bit for migration entries and get at
>> alteast the most relevant 64bit architectures supported easily.
> 
> Yes, but I think having two mechanisms for the single problem can confuse
> people.
> 

It would be one bit with two different meanings depending on the swp type.

> IIUC the swap bit is already defined in major archs anyway, and since anon
> exclusive bit is best-effort (or am I wrong?..), I won't worry too much on

It kind-of is best effort, but the goal is to have all archs support it.

... just like the young bit here?

> archs outside x86/arm/ppc/s390 on having anon exclusive bit lost during
> migrations, because afaict the whole swap type of ANON_EXCLUSIVE_READ is
> only servicing that very minority.. which seems to be a pity to waste the

I have a big item on my todo list to support all, but I have different
priorities right now.

If there is no free bit, simply steal one from the offset ... which is
the same thing your approach would do, just in a different way, no?

> swp type on all archs even if the archs defined swp pte bits just for anon
> exclusive.

Why do we care? We walk about one type not one bit.

-- 
Thanks,

David / dhildenb