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

[Baoquan He]

On 10/15/24 at 12:13pm, Ryan Roberts wrote:
> 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@xxxxxxx>
> >> ---
> >>
> >> ***NOTE***
> >> Any confused maintainers may want to read the cover note here for context:
> >> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@xxxxxxx/
> >>
> >>  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.

Makes sense, thanks for your explanation.
> 
> 
> 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@xxxxxxxxxx>
> 
> Thanks!
> 
> > 
> >>  
> >>  	crash_notes = __alloc_percpu(size, align);
> >>  	if (!crash_notes) {
> >> -- 
> >> 2.43.0
> >>
> >>
> > 
>