netdev - Re: [PATCH net-next v2 2/4] page_pool: add interface to manipulate frag count in page pool

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Date:   Thu, 12 Aug 2021 17:17:05 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     brouer@...hat.com, alexander.duyck@...il.com,
        linux@...linux.org.uk, mw@...ihalf.com, linuxarm@...neuler.org,
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        thomas.petazzoni@...tlin.com, hawk@...nel.org,
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Subject: Re: [PATCH net-next v2 2/4] page_pool: add interface to manipulate
 frag count in page pool


On 06/08/2021 04.46, Yunsheng Lin wrote:
> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
> +							  long nr)
> +{
> +	long ret;
> +
> +	/* As suggested by Alexander, atomic_long_read() may cover up the
> +	 * reference count errors, so avoid calling atomic_long_read() in
> +	 * the cases of freeing or draining the page_frags, where we would
> +	 * not expect it to match or that are slowpath anyway.
> +	 */
> +	if (__builtin_constant_p(nr) &&
> +	    atomic_long_read(&page->pp_frag_count) == nr)
> +		return 0;
> +
> +	ret = atomic_long_sub_return(nr, &page->pp_frag_count);
> +	WARN_ON(ret < 0);

I worried about this WARN_ON() as it generates an 'ud2' instruction 
which influence I-cache fetching.  But I have disassembled (objdump) the 
page_pool.o binary and the ud2 gets placed last in the main function 
page_pool_put_page() that use this inlined function.
Thus, I assume this is not a problem :-)


> +	return ret;