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

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Message-ID: <aQ4jmguL-dwKea-N@fedora>
Date: Fri, 7 Nov 2025 08:51:38 -0800
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: 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, Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one_*()
 about HIGHMEM

+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.

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.