[PATCH v4 3/4] mm/percpu: Make cached pages lookup explicit

[Kaitao Cheng <kaitao.cheng@linux.dev>]

From: Kaitao Cheng <chengkaitao@kylinos.cn>

pcpu_depopulate_chunk() only needs the temporary pages array that was
already allocated by an earlier successful population attempt. Passing
GFP_KERNEL to pcpu_get_pages() in this path is misleading because the
depopulation path is not expected to allocate the array.

Teach pcpu_get_pages() to treat a zero gfp mask as a cached-only lookup
and add pcpu_get_pages_cached() for that use case. This keeps allocation
on the populate path tied to the caller supplied GFP mask while making
the depopulate path's dependency on the cached array explicit.

Suggested-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
 mm/percpu-vm.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
index ccd03cc152d4..7ed216192fc0 100644
--- a/mm/percpu-vm.c
+++ b/mm/percpu-vm.c
@@ -21,7 +21,8 @@ static struct page *pcpu_chunk_page(struct pcpu_chunk *chunk,
 
 /**
  * pcpu_get_pages - get temp pages array
- * @gfp: allocation flags passed to the underlying allocator
+ * @gfp: allocation flags passed to the underlying allocator, 0 to only
+ *	 return the cached array
  *
  * Returns pointer to array of pointers to struct page which can be indexed
  * with pcpu_page_idx().  Note that there is only one array and accesses
@@ -37,11 +38,16 @@ static struct page **pcpu_get_pages(gfp_t gfp)
 
 	lockdep_assert_held(&pcpu_alloc_mutex);
 
-	if (!pages)
+	if (!pages && gfp)
 		pages = pcpu_mem_zalloc(pages_size, gfp);
 	return pages;
 }
 
+static struct page **pcpu_get_pages_cached(void)
+{
+	return pcpu_get_pages(0);
+}
+
 /**
  * pcpu_free_pages - free pages which were allocated for @chunk
  * @chunk: chunk pages were allocated for
@@ -333,7 +339,7 @@ static void pcpu_depopulate_chunk(struct pcpu_chunk *chunk,
 	 * successful population attempt so the temp pages array must
 	 * be available now.
 	 */
-	pages = pcpu_get_pages(GFP_KERNEL);
+	pages = pcpu_get_pages_cached();
 	BUG_ON(!pages);
 
 	/* unmap and free */
-- 
2.50.1 (Apple Git-155)