linux-kernel - Re: [RFC 3/8] mm: Avoid using set_page_count() in set_page

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Message-ID: <7b131cb1-68d8-6746-f9c1-2b01d4838869@nvidia.com>
Date:   Tue, 26 Oct 2021 10:53:24 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-m68k@...ts.linux-m68k.org, anshuman.khandual@....com,
        willy@...radead.org, akpm@...ux-foundation.org,
        william.kucharski@...cle.com, mike.kravetz@...cle.com,
        vbabka@...e.cz, geert@...ux-m68k.org, schmitzmic@...il.com,
        rostedt@...dmis.org, mingo@...hat.com, hannes@...xchg.org,
        guro@...com, songmuchun@...edance.com, weixugc@...gle.com,
        gthelen@...gle.com
Subject: Re: [RFC 3/8] mm: Avoid using set_page_count() in
 set_page_recounted()

On 10/26/21 10:38, Pasha Tatashin wrote:
> set_page_refcounted() converts a non-refcounted page that has
> (page->_refcount == 0) into a refcounted page by setting _refcount to 1,
> 
> Use page_ref_inc_return() instead to avoid unconditionally overwriting
> the _refcount value.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>   mm/internal.h | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index cf3cb933eba3..cf345fac6894 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -91,9 +91,12 @@ static inline bool page_evictable(struct page *page)
>    */
>   static inline void set_page_refcounted(struct page *page)
>   {
> +	int refcnt;
> +
>   	VM_BUG_ON_PAGE(PageTail(page), page);
>   	VM_BUG_ON_PAGE(page_ref_count(page), page);
> -	set_page_count(page, 1);
> +	refcnt = page_ref_inc_return(page);
> +	VM_BUG_ON_PAGE(refcnt != 1, page);

Hi Pavel,

I am acutely uncomfortable with this change, because it changes the
meaning and behavior of the function to something completely different,
while leaving the function name unchanged. Furthermore, in relies upon
debug assertions, rather than a return value (for example) to verify
that all is well.

I understand where this patchset is going, but this intermediate step is
not a good move.

Also, for the overall series, if you want to change from
"set_page_count()" to "inc_and_verify_val_equals_one()", then the way to
do that is *not* to depend solely on VM_BUG*() to verify. Instead,
return something like -EBUSY if incrementing the value results in a
surprise, and let the caller decide how to handle it.

thanks,
-- 
John Hubbard
NVIDIA

>   }
>   
>   extern unsigned long highest_memmap_pfn;
>