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

Ryan Roberts
2024-10-15T11:22:37Z

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
>>
>>
>