linux-kernel - Re: [PATCH 03/21] x86/mm: factor out phys_pgd

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Message-ID: <20251025114801.GWaPy48dhlZ_EVoqKi@fat_crate.local>
Date: Sat, 25 Oct 2025 13:48:01 +0200
From: Borislav Petkov <bp@...en8.de>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Andy Lutomirski <luto@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Yuanchu Xie <yuanchu@...gle.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>, peterz@...radead.org,
	dave.hansen@...ux.intel.com, mingo@...hat.com, tglx@...utronix.de,
	akpm@...ux-foundation.org, david@...hat.com, derkling@...gle.com,
	junaids@...gle.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, reijiw@...gle.com, rientjes@...gle.com,
	rppt@...nel.org, vbabka@...e.cz, x86@...nel.org,
	yosry.ahmed@...ux.dev
Subject: Re: [PATCH 03/21] x86/mm: factor out phys_pgd_init()

On Wed, Sep 24, 2025 at 02:59:38PM +0000, Brendan Jackman wrote:
> +static unsigned long __meminit
> +__kernel_physical_mapping_init(unsigned long paddr_start,
> +			       unsigned long paddr_end,
> +			       unsigned long page_size_mask,
> +			       pgprot_t prot, bool init)
> +{
> +	bool pgd_changed;

I have to say, that pgd_changed is yuck but I don't have a better idea and
this has happened a long time ago anyway.

How about you have the caller pass in false:

	bool pgd_changed = false;

and then callee sets it to true when it does so?

> +	unsigned long paddr_last;

The tip-tree preferred ordering of variable declarations at the
beginning of a function is reverse fir tree order::

	struct long_struct_name *descriptive_name;
	unsigned long foo, bar;
	unsigned int tmp;
	int ret;

The above is faster to parse than the reverse ordering::

	int ret;
	unsigned int tmp;
	unsigned long foo, bar;
	struct long_struct_name *descriptive_name;

And even more so than random ordering::

	unsigned long foo, bar;
	int ret;
	struct long_struct_name *descriptive_name;
	unsigned int tmp;

> +
> +	paddr_last = phys_pgd_init(init_mm.pgd, paddr_start, paddr_end, page_size_mask,
> +				   prot, init, &pgd_changed);
> +	if (pgd_changed)
> +		sync_global_pgds((unsigned long)__va(paddr_start),
> +				 (unsigned long)__va(paddr_end) - 1);
> +
> +	return paddr_last;
> +}
>  
>  /*
>   * Create page table mapping for the physical memory for specific physical
> 
> -- 
> 2.50.1
> 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette