linux-kernel - Re: [PATCH v3 5/9] riscv: mm: Add memory hotplugging support

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Date: Tue, 21 May 2024 16:20:31 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Björn Töpel <bjorn@...nel.org>,
	Albert Ou <aou@...s.berkeley.edu>,
	David Hildenbrand <david@...hat.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	linux-riscv@...ts.infradead.org,
	Björn Töpel <bjorn@...osinc.com>,
	Andrew Bresticker <abrestic@...osinc.com>,
	Chethan Seshadri <Chethan.Seshadri@...alinasystems.io>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Santosh Mamila <santosh.mamila@...alinasystems.io>,
	Sivakumar Munnangi <siva.munnangi@...alinasystems.io>,
	Sunil V L <sunilvl@...tanamicro.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 5/9] riscv: mm: Add memory hotplugging support

On Tue, May 21, 2024 at 03:19:37PM +0200, Alexandre Ghiti wrote:
> On Tue, May 21, 2024 at 1:49 PM Björn Töpel <bjorn@...nel.org> wrote:
> > +       if (PageReserved(page)) {
> > +               __ClearPageReserved(page);
> 
> What's the difference between __ClearPageReserved() and
> ClearPageReserved()? Because it seems like free_reserved_page() calls
> the latter already, so why would you need to call
> __ClearPageReserved() on the first page?

__{Set,Clear}Page are the non-atomic version.
Usually used when you know that no one else can fiddle with the page, which
should be the case here since we are removing the memory.

As to why we have __ClearPageReserved and then having
free_reserved_page() call ClearPageReserved I do not really know.
Looking at the history, it has always been like this.

I remember I looked at this a few years ago but I cannot remember the outcome
of that.

Maybe David remembers better, but I think we could remove that
__ClearPageReserved.
Looking at powerpc implementation code, it does not do the
__ClearPageReserved and relies only on free_reserved_page().

I will have a look.

-- 
Oscar Salvador
SUSE Labs