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 subtract
the number here. The number of pages will be added back with
pcpu_chunk_populated() when populating pages.

Adding code comment to make that more understandable.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

diff --git a/mm/percpu.c b/mm/percpu.c
index 68d5ba6..2a7313b 100644 (file)
--- 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 added back with pcpu_chunk_populated()
+        * when populating pages.
+        */
        if (nr_empty_pages)
                pcpu_update_empty_pages(chunk, -nr_empty_pages);