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From: Vincent Donnefort <vdonnefort@google.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: maz@kernel.org, oliver.upton@linux.dev, kvmarm@lists.linux.dev,
	linux-arm-kernel@lists.infradead.org, kernel-team@android.com,
	will@kernel.org
Subject: Re: [PATCH v1 1/2] KVM: arm64: Do not transfer page refcount for THP adjustment
Date: Wed, 27 Sep 2023 13:40:42 +0100	[thread overview]
Message-ID: <ZRQiypIrEVFAo_3Y@google.com> (raw)
In-Reply-To: <ZRPTDJwcImUTHbm+@casper.infradead.org>

On Wed, Sep 27, 2023 at 08:00:28AM +0100, Matthew Wilcox wrote:
> On Tue, Sep 26, 2023 at 07:19:31PM +0100, Vincent Donnefort wrote:
> > With folios, GUP affects a refcount common to all pages forming the THP.
> > There is therefore no need to move the refcount from the tail to the
> > head page. Under the hood it decrements and increments
> > the same counter.
> 
> FYI, this isn't new to folios.  get_page() has _always_ operated on the
> head page of a compound page.  Here's how it worked in 2.6.12-rc2:
> 
> +static inline void get_page(struct page *page)
> +{
> +       if (unlikely(PageCompound(page)))
> +               page = (struct page *)page->private;
> +       atomic_inc(&page->_count);
> +}
> 
> I can't be bothered to switch over to the linux-fullhistory tree and
> show how it evolved, because i think this sufficiently makes the point --
> you can backport this patch as far as you like.

Thanks for having a look!

I'll refine that description in the next version.

> 
> Folios just make it obvious that this is what's going on.  And that's a
> good thing!  Making the MM less inscrutable is a win for everybody.

WARNING: multiple messages have this Message-ID (diff)
From: Vincent Donnefort <vdonnefort@google.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: maz@kernel.org, oliver.upton@linux.dev, kvmarm@lists.linux.dev,
	linux-arm-kernel@lists.infradead.org, kernel-team@android.com,
	will@kernel.org
Subject: Re: [PATCH v1 1/2] KVM: arm64: Do not transfer page refcount for THP adjustment
Date: Wed, 27 Sep 2023 13:40:42 +0100	[thread overview]
Message-ID: <ZRQiypIrEVFAo_3Y@google.com> (raw)
In-Reply-To: <ZRPTDJwcImUTHbm+@casper.infradead.org>

On Wed, Sep 27, 2023 at 08:00:28AM +0100, Matthew Wilcox wrote:
> On Tue, Sep 26, 2023 at 07:19:31PM +0100, Vincent Donnefort wrote:
> > With folios, GUP affects a refcount common to all pages forming the THP.
> > There is therefore no need to move the refcount from the tail to the
> > head page. Under the hood it decrements and increments
> > the same counter.
> 
> FYI, this isn't new to folios.  get_page() has _always_ operated on the
> head page of a compound page.  Here's how it worked in 2.6.12-rc2:
> 
> +static inline void get_page(struct page *page)
> +{
> +       if (unlikely(PageCompound(page)))
> +               page = (struct page *)page->private;
> +       atomic_inc(&page->_count);
> +}
> 
> I can't be bothered to switch over to the linux-fullhistory tree and
> show how it evolved, because i think this sufficiently makes the point --
> you can backport this patch as far as you like.

Thanks for having a look!

I'll refine that description in the next version.

> 
> Folios just make it obvious that this is what's going on.  And that's a
> good thing!  Making the MM less inscrutable is a win for everybody.

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  reply	other threads:[~2023-09-27 12:40 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-09-26 18:19 [PATCH v1 0/2] KVM: arm64: Use folio for THP support Vincent Donnefort
2023-09-26 18:19 ` Vincent Donnefort
2023-09-26 18:19 ` [PATCH v1 1/2] KVM: arm64: Do not transfer page refcount for THP adjustment Vincent Donnefort
2023-09-26 18:19   ` Vincent Donnefort
2023-09-27  7:00   ` Matthew Wilcox
2023-09-27  7:00     ` Matthew Wilcox
2023-09-27 12:40     ` Vincent Donnefort [this message]
2023-09-27 12:40       ` Vincent Donnefort
2023-09-26 18:19 ` [PATCH v1 2/2] KVM: arm64: Use folio " Vincent Donnefort
2023-09-26 18:19   ` Vincent Donnefort
2023-09-27  6:30 ` [PATCH v1 0/2] KVM: arm64: Use folio for THP support Oliver Upton
2023-09-27  6:30   ` Oliver Upton
2023-09-27 12:39   ` Vincent Donnefort
2023-09-27 12:39     ` Vincent Donnefort

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