Re: [RFC PATCH v1 19/57] crash: Remove PAGE_SIZE compile-time constant assumption

From: Ryan Roberts <ryan.roberts@arm.com>
To: Baoquan He <bhe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	David Hildenbrand <david@redhat.com>,
	Greg Marsden <greg.marsden@oracle.com>,
	Ivan Ivanov <ivan.ivanov@suse.com>,
	Kalesh Singh <kaleshsingh@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Matthias Brugger <mbrugger@suse.com>,
	Miroslav Benes <mbenes@suse.cz>, Will Deacon <will@kernel.org>,
	kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v1 19/57] crash: Remove PAGE_SIZE compile-time constant assumption
Date: Tue, 15 Oct 2024 12:13:55 +0100	[thread overview]
Message-ID: <2e561511-ab20-4aa9-9b92-bd6ac6678087@arm.com> (raw)
In-Reply-To: <Zw3luHGG3LqHge2m@MiWiFi-R3L-srv>

On 15/10/2024 04:47, Baoquan He wrote:
> On 10/14/24 at 11:58am, Ryan Roberts wrote:
>> To prepare for supporting boot-time page size selection, refactor code
>> to remove assumptions about PAGE_SIZE being compile-time constant. Code
>> intended to be equivalent when compile-time page size is active.
>>
>> Updated BUILD_BUG_ON() to test against limit.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>
>> ***NOTE***
>> Any confused maintainers may want to read the cover note here for context:
>> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>>
>>  kernel/crash_core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index 63cf89393c6eb..978c600a47ac8 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -465,7 +465,7 @@ static int __init crash_notes_memory_init(void)
>>  	 * Break compile if size is bigger than PAGE_SIZE since crash_notes
>>  	 * definitely will be in 2 pages with that.
>>  	 */
>> -	BUILD_BUG_ON(size > PAGE_SIZE);
>> +	BUILD_BUG_ON(size > PAGE_SIZE_MIN);
> 
> This should be OK. While one thing which could happen is if selected size
> is 64K, PAGE_SIZE_MIN is 4K, it will issue a false-positive warning when
> compiling while actual it's not a problem during running. 

PAGE_SIZE can only ever be bigger than PAGE_SIZE_MIN if compiling a "boot-time
page size" build. And in this case, you need to know that size is small enough
to work with any of the boot-time selectable page sizes. Since size
(=sizeof(note_buf_t)) is invariant to PAGE_SIZE, we can do this by checking
against PAGE_SIZE_MIN.

So I don't think this could ever lead to a false-positive.

Not sure if
> that could happen on arm64. Anyway, we can check the crash_notes to get
> why it's so big when it really happens. So,
> 
> Acked-by: Baoquan He <bhe@redhat.com>

Thanks!

> 
>>  
>>  	crash_notes = __alloc_percpu(size, align);
>>  	if (!crash_notes) {
>> -- 
>> 2.43.0
>>
>>
> 

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