linux-kernel - Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one

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Message-ID: <aRBGFZW09oFxM4k5@kernel.org>
Date: Sun, 9 Nov 2025 09:43:17 +0200
From: Mike Rapoport <rppt@...nel.org>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: Arnd Bergmann <arnd@...db.de>, Huacai Chen <chenhuacai@...ngson.cn>,
	Huacai Chen <chenhuacai@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kevin Brodsky <kevin.brodsky@....com>, Jan Kara <jack@...e.cz>,
	linux-mm@...ck.org, Linux-Arch <linux-arch@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one_*()
 about HIGHMEM

On Fri, Nov 07, 2025 at 08:51:38AM -0800, Vishal Moola (Oracle) wrote:
> +Cc: Mike
> 
> On Fri, Nov 07, 2025 at 12:21:38PM +0100, Arnd Bergmann wrote:
> > On Fri, Nov 7, 2025, at 10:59, Huacai Chen wrote:
> > > __{pgd,p4d,pud,pmd,pte}_alloc_one_*() always allocate pages with GFP
> > > flag GFP_PGTABLE_KERNEL/GFP_PGTABLE_USER. These two macros are defined
> > > as follows:
> > >
> > >  #define GFP_PGTABLE_KERNEL	(GFP_KERNEL | __GFP_ZERO)
> > >  #define GFP_PGTABLE_USER	(GFP_PGTABLE_KERNEL | __GFP_ACCOUNT)
> > >
> > > There is no __GFP_HIGHMEM in them, so we needn't to clear __GFP_HIGHMEM
> > > explicitly.
> > >
> > > Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> > > ---
> > > Resend because the lines begin with # was eaten by git.
> > 
> > Thanks for your patch, this is an area I've also started
> > looking at, with the intention to reduce the references
> > to __GFO_HIGHMEM to the minimum we need for supporting the
> > remaining platforms that need to use highmem somewhere.
> 
> Yay! Thanks for doing that, I like less highmem :)
> 
> > I'm not sure what the reason is for your patch, I assume
> > this is meant purely as a cleanup, correct? Are you looking
> > at a wider set of related cleanups, or did you just notice
> > this one instance?
> > 
> > Note that for the moment, the 32-bit arm __pte_alloc_one() function
> > still passes __GFP_HIGHMEM when CONFIG_HIGHPTE is set, though
> > I would like to remove that code path. Unless we remove
> > that at the same time, this should probably be explained in your
> > patch description.
> 
> Skimming the functions, __pte_alloc_one_kernel() doesn't get passed in
> a gfp, while __pte_alloc_one() does. IOW I __pte_alloc_one_kernel()
> cares about architecture gfp, while the latter does care - so they are
> 2 very different cases.

__pte_alloc_one() has gfp parameter to accommodate CONFIG_HIGHPTE that x86
used to have until quite recently and arm still has.
 
> Might be helpful to explain, although I don't think it matters much.
> 
> I've cc-ed Mike, he might have more useful opinions these functions.
> 

-- 
Sincerely yours,
Mike.