linux-kernel - Re: [PATCH 4/8] mm/percpu: add comment to state the empty populated pages accounting

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Message-ID: <Y1bDtmfCee+V32Ml@fedora>
Date:   Mon, 24 Oct 2022 09:56:22 -0700
From:   Dennis Zhou <dennis@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH 4/8] mm/percpu: add comment to state the empty populated
 pages accounting

On Mon, Oct 24, 2022 at 04:14:31PM +0800, Baoquan He wrote:
> When allocating an area from a chunk, pcpu_block_update_hint_alloc()
> is called to update chunk metadata, including chunk's and global
> nr_empty_pop_pages. However, if the allocation is not atomic, some
> blocks may not be populated with pages yet, while we still account it
> here. The number of pages will be subtracted with pcpu_chunk_populated()
> when populating pages.
> 
> Adding code comment to make that more understandable.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/percpu.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index a8121302a79c..09e407338573 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -831,13 +831,15 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
>  
>  	/*
>  	 * Update s_block.
> -	 * block->first_free must be updated if the allocation takes its place.
> -	 * If the allocation breaks the contig_hint, a scan is required to
> -	 * restore this hint.
>  	 */
>  	if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
>  		nr_empty_pages++;
>  
> +	/*
> +	 * block->first_free must be updated if the allocation takes its place.
> +	 * If the allocation breaks the contig_hint, a scan is required to
> +	 * restore this hint.
> +	 */
>  	if (s_off == s_block->first_free)
>  		s_block->first_free = find_next_zero_bit(
>  					pcpu_index_alloc_map(chunk, s_index),
> @@ -912,6 +914,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
>  		}
>  	}
>  
> +	/*
> +	 * If the allocation is not atomic, some blocks may not
> +	 * be populated with pages, while we account it here.
> +	 * The number of pages will be subtracted with
> +	 * pcpu_chunk_populated() when populating pages.
> +	 */
>  	if (nr_empty_pages)
>  		pcpu_update_empty_pages(chunk, -nr_empty_pages);
>  
> -- 
> 2.34.1
> 

Heh, that's a little more subtle than I remember it being.

Acked-by: Dennis Zhou <dennis@...nel.org>

Thanks,
Dennis