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

Baoquan He
2022-10-24T08:16:52Z

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