[PATCH RFC net-next/mm V4 1/2] mm/page_pool: catch page_pool memory leaks

[Jesper Dangaard Brouer <brouer@redhat.com>]

Pages belonging to a page_pool (PP) instance must be freed through the
PP APIs in-order to correctly release any DMA mappings and release
refcnt on the DMA device when freeing PP instance. When PP release a
page (page_pool_release_page) the page->pp_magic value is cleared.

This patch detect a leaked PP page in free_page_is_bad() via
unexpected state of page->pp_magic value being PP_SIGNATURE.

We choose to report and treat it as a bad page. It would be possible
to release the page via returning it to the PP instance as the
page->pp pointer is likely still valid.

Notice this code is only activated when either compiled with
CONFIG_DEBUG_VM or boot cmdline debug_pagealloc=on, and
CONFIG_PAGE_POOL.

Reduced example output of leak with PP_SIGNATURE = dead000000000040:

 BUG: Bad page state in process swapper/0  pfn:110bbf
 page:000000005bc8cfb8 refcount:0 mapcount:0 mapping:0000000000000000 index:0x110bbf000 pfn:0x110bbf
 flags: 0x2fffff80000000(node=0|zone=2|lastcpupid=0x1fffff)
 raw: 002fffff80000000 dead000000000040 ffff888117255000 0000000000000000
 raw: 0000000110bbf000 000000000000003e 00000000ffffffff 0000000000000000
 page dumped because: page_pool leak
 [...]

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 mm/page_alloc.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 47421bedc12b..e6b996da39d4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1085,6 +1085,9 @@ static inline bool page_expected_state(struct page *page,
 			page_ref_count(page) |
 #ifdef CONFIG_MEMCG
 			page->memcg_data |
+#endif
+#ifdef CONFIG_PAGE_POOL
+			((page->pp_magic & ~0x3UL) == PP_SIGNATURE) |
 #endif
 			(page->flags & check_flags)))
 		return false;
@@ -1111,6 +1114,10 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
 #ifdef CONFIG_MEMCG
 	if (unlikely(page->memcg_data))
 		bad_reason = "page still charged to cgroup";
+#endif
+#ifdef CONFIG_PAGE_POOL
+	if (unlikely((page->pp_magic & ~0x3UL) == PP_SIGNATURE))
+		bad_reason = "page_pool leak";
 #endif
 	return bad_reason;
 }