linux-kernel - Re: [PATCH v4 7/7] mm/mm_init: Use for_each_valid_pfn() in init_unavailable

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Message-ID: <cabc322e-d5ab-4371-a506-c7809717b38b@redhat.com>
Date: Fri, 25 Apr 2025 18:17:25 +0200
From: David Hildenbrand <david@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>, Mike Rapoport <rppt@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Sauerwein, David" <dssauerw@...zon.de>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mike Rapoport <rppt@...ux.ibm.com>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Ruihan Li <lrh2000@....edu.cn>
Subject: Re: [PATCH v4 7/7] mm/mm_init: Use for_each_valid_pfn() in
 init_unavailable_range()

On 23.04.25 15:33, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Currently, memmap_init initializes pfn_hole with 0 instead of
> ARCH_PFN_OFFSET. Then init_unavailable_range will start iterating each
> page from the page at address zero to the first available page, but it
> won't do anything for pages below ARCH_PFN_OFFSET because pfn_valid
> won't pass.
> 
> If ARCH_PFN_OFFSET is very large (e.g., something like 2^64-2GiB if the
> kernel is used as a library and loaded at a very high address), the
> pointless iteration for pages below ARCH_PFN_OFFSET will take a very
> long time, and the kernel will look stuck at boot time.
> 
> Use for_each_valid_pfn() to skip the pointless iterations.
> 
> Reported-by: Ruihan Li <lrh2000@....edu.cn>
> Suggested-by: Mike Rapoport <rppt@...nel.org>
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Tested-by: Ruihan Li <lrh2000@....edu.cn>
> ---
>   mm/mm_init.c | 6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 41884f2155c4..0d1a4546825c 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -845,11 +845,7 @@ static void __init init_unavailable_range(unsigned long spfn,
>   	unsigned long pfn;
>   	u64 pgcnt = 0;
>   
> -	for (pfn = spfn; pfn < epfn; pfn++) {
> -		if (!pfn_valid(pageblock_start_pfn(pfn))) {
> -			pfn = pageblock_end_pfn(pfn) - 1;
> -			continue;
> -		}

So, if the first pfn in a pageblock is not valid, we skip the whole 
pageblock ...

> +	for_each_valid_pfn(pfn, spfn, epfn) {
>   		__init_single_page(pfn_to_page(pfn), pfn, zone, node);
>   		__SetPageReserved(pfn_to_page(pfn));
>   		pgcnt++;

but here, we would process further pfns inside such a pageblock?

-- 
Cheers,

David / dhildenb