Re: [PATCH mm-unstable] mm: move folio_set_compound_order() to mm/internal.h

[John Hubbard]

On 12/16/22 14:27, Andrew Morton wrote:
> On Tue, 13 Dec 2022 13:20:53 -0800 Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx> wrote:
>
> > folio_set_compound_order() is moved to an mm-internal location so external
> > folio users cannot misuse this function. Change the name of the function
> > to folio_set_order() and use WARN_ON_ONCE() rather than BUG_ON. Also,
> > handle the case if a non-large folio is passed and add clarifying comments
> > to the function.
>
> This differs from the version I previously merged:
>
> --- a/mm/internal.h~mm-move-folio_set_compound_order-to-mm-internalh-update
> +++ a/mm/internal.h
> @@ -384,8 +384,10 @@ int split_free_page(struct page *free_pa
>    */
>   static inline void folio_set_order(struct folio *folio, unsigned int order)
>   {
> -	if (WARN_ON_ONCE(!folio_test_large(folio)))
> +	if (!folio_test_large(folio)) {
> +		WARN_ON_ONCE(order);
>   		return;
> +	}

I think that's out of date?

We eventually settled on the version that is (as of this a few minutes
ago) already in mm-unstable (commit fdea060a130d: "mm: move
folio_set_compound_order() to mm/internal.h"), which has it like this:

       if (WARN_ON_ONCE(!folio_test_large(folio)))
               return;

>   
>   	folio->_folio_order = order;
>   #ifdef CONFIG_64BIT
>
> Makes sense.  But wouldn't
>
> 	if (WARN_ON_ONCE(order && !folio_test_large(folio)))
>
> be clearer?

That's a little narrower of a check. But maybe that's desirable. Could
someone (Mike, Muchun, Sidhartha) comment on which behavior is
preferable, please? I think I'm a little dizzy at this point. :)


thanks,
--
John Hubbard
NVIDIA