linux-kernel - Re: [PATCH mm-unstable] mm: move folio_set_compound

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Message-ID: <963ba9b4-6ddf-39bc-85cf-2feef542029d@nvidia.com>
Date:   Fri, 16 Dec 2022 14:56:47 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>
CC:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <songmuchun@...edance.com>, <mike.kravetz@...cle.com>,
        <willy@...radead.org>
Subject: Re: [PATCH mm-unstable] mm: move folio_set_compound_order() to
 mm/internal.h

On 12/16/22 14:27, Andrew Morton wrote:
> On Tue, 13 Dec 2022 13:20:53 -0800 Sidhartha Kumar <sidhartha.kumar@...cle.com> 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I think that's out of date?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