Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for our internal use

From: Chao Yu
Date: Mon Jul 12 2021 - 20:46:43 EST

On 2021/7/12 14:53, Michal Hocko wrote:
On Sat 10-07-21 16:11:38, Chao Yu wrote:
On 2021/7/7 17:57, Mel Gorman wrote:
I think it would work but it would be preferable to find out why the
tail page has an order set in the first place. I've looked over


mm/page_alloc.c and mm/compaction.c a few times and did not spot where
set_private_page(page, 0) is missed when it should be covered by
clear_page_guard or del_page_from_free_list :(

I didn't enable CONFIG_DEBUG_PAGEALLOC, so we will expect page private
should be cleared by del_page_from_free_list(), but I guess it only clears
the buddy's private field rather than original page's, so I added below
diff and check the dmesg, it looks stall private value in original page
will be left commonly... Let me know if I missed something?

Page private should be cleared when the page is freed to the allocator.

Quoted from Jaegeuk's comments in [1]

"Hmm, I can see it in 4.14 and 5.10 kernel.

The trace is on:

30875 [ 1065.118750] c3 87 f2fs_migrate_page+0x354/0x45c
30876 [ 1065.123872] c3 87 move_to_new_page+0x70/0x30c
30877 [ 1065.128813] c3 87 migrate_pages+0x3a0/0x964
30878 [ 1065.133583] c3 87 compact_zone+0x608/0xb04
30879 [ 1065.138257] c3 87 kcompactd+0x378/0x4ec
30880 [ 1065.142664] c3 87 kthread+0x11c/0x12c
30881 [ 1065.146897] c3 87 ret_from_fork+0x10/0x18

It seems compaction_alloc() gets a free page which doesn't reset the fields?"

So problem here we met is: in f2fs_migrate_page(), newpage may has stall .private
value rather than PG_private flag, which may cause f2fs will treat the page with
wrong private status.

mm/page_alloc.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a06bcfe6f786..1e7031ff548e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1029,6 +1029,7 @@ static inline void __free_one_page(struct page *page,
unsigned long combined_pfn;
unsigned int max_order;
struct page *buddy;
+ struct page *orig_page = page;
bool to_tail;

max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);
@@ -1097,6 +1098,10 @@ static inline void __free_one_page(struct page *page,

set_buddy_order(page, order);
+ if (orig_page != page) {
+ if (WARN_ON_ONCE(orig_page->private))
+ pr_info("2order:%x, origpage.private:%x", order, orig_page->private);
+ }

Why is this expected? Buddy allocator uses page private to store order.
Whether we are merging to the freed page or coalesce it to a different

The order was only set in head page, right? Since it looks __free_one_page() tries
to clear page.private for every buddy with del_page_from_free_list().

If that is true, after done_merging label in __free_one_page, if original page is
a tail page, we may missed to clear its page.private field?


page is not all that important.