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

Dennis Zhou
2022-10-24T18:17:53Z

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@redhat.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@kernel.org>

Thanks,
Dennis