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

From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Huang Ying <ying.huang@intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Nadav Amit <nadav.amit@gmail.com>,
	Hugh Dickins <hughd@google.com>, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH RFC 0/4] mm: Remember young bit for migration entries
Date: Tue, 2 Aug 2022 22:23:49 +0200	[thread overview]
Message-ID: <24ffea6e-ca66-2b94-c682-48a42a655fd1@redhat.com> (raw)
In-Reply-To: <YumFs0jpCc/Mwjzf@xz-m1.local>

> 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