linux-kernel - Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP

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Date: Wed, 31 Jan 2024 11:30:56 +0100
From: David Hildenbrand <david@...hat.com>
To: Yin Fengwei <fengwei.yin@...el.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
 Nick Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>,
 Michael Ellerman <mpe@...erman.id.au>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP

On 31.01.24 03:30, Yin Fengwei wrote:
> 
> 
> On 1/29/24 22:32, David Hildenbrand wrote:
>> +static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
>> +		unsigned long addr, pte_t *ptep, unsigned int nr, int full)
>> +{
>> +	pte_t pte, tmp_pte;
>> +
>> +	pte = ptep_get_and_clear_full(mm, addr, ptep, full);
>> +	while (--nr) {
>> +		ptep++;
>> +		addr += PAGE_SIZE;
>> +		tmp_pte = ptep_get_and_clear_full(mm, addr, ptep, full);
>> +		if (pte_dirty(tmp_pte))
>> +			pte = pte_mkdirty(pte);
>> +		if (pte_young(tmp_pte))
>> +			pte = pte_mkyoung(pte);
> I am wondering whether it's worthy to move the pte_mkdirty() and pte_mkyoung()
> out of the loop and just do it one time if needed. The worst case is that they
> are called nr - 1 time. Or it's just too micro?

I also thought about just indicating "any_accessed" or "any_dirty" using 
flags to the caller, to avoid the PTE modifications completely. Felt a 
bit micro-optimized.

Regarding your proposal: I thought about that as well, but my assumption 
was that dirty+young are "cheap" to be set.

On x86, pte_mkyoung() is setting _PAGE_ACCESSED.
pte_mkdirty() is setting _PAGE_DIRTY | _PAGE_SOFT_DIRTY, but it also has 
to handle the saveddirty handling, using some bit trickery.

So at least for pte_mkyoung() there would be no real benefit as far as I 
can see (might be even worse). For pte_mkdirty() there might be a small 
benefit.

Is it going to be measurable? Likely not.

Am I missing something?

Thanks!

-- 
Cheers,

David / dhildenb