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

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Date:   Mon, 24 Oct 2022 16:14:31 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Baoquan He <bhe@...hat.com>
Subject: [PATCH 4/8] mm/percpu: add comment to state the empty populated pages accounting

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