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

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Message-ID: <91CA8854-2E86-4AF3-BAD0-8C47833F59D4@infradead.org>
Date: Fri, 25 Apr 2025 20:08:50 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: David Hildenbrand <david@...hat.com>, 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 25 April 2025 17:17:25 BST, David Hildenbrand <david@...hat.com> wrote:
>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?
>

Is it not the case that either *all*, or *none*, of the PFNs within a given pageblock will be valid? 

I assumed that was *why* it had that skip, as an attempt at the kind of optimisation that for_each_valid_pfn() now gives us?