Re: [PATCH v2 1/5] mm: Separate folio_split_memcg_refs() from split_page_memcg()

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On 15.03.25 00:15, Zi Yan wrote:
On 14 Mar 2025, at 17:49, David Hildenbrand wrote:

On 14.03.25 14:36, Matthew Wilcox (Oracle) wrote:
Folios always use memcg_data to refer to the mem_cgroup while pages
allocated with GFP_ACCOUNT have a pointer to the obj_cgroup.  Since the
caller already knows what it has, split the function into two and then
we don't need to check.

Move the assignment of split folio memcg_data to the point where we set
up the other parts of the new folio.  That leaves folio_split_memcg_refs()
just handling the memcg accounting.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Acked-by: Zi Yan <ziy@xxxxxxxxxx>
---
   include/linux/memcontrol.h |  7 +++++++
   mm/huge_memory.c           | 16 ++++------------
   mm/memcontrol.c            | 17 +++++++++++++----
   3 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 57664e2a8fb7..d090089c5497 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1039,6 +1039,8 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
   }
    void split_page_memcg(struct page *head, int old_order, int new_order);
+void folio_split_memcg_refs(struct folio *folio, unsigned old_order,
+		unsigned new_order);
    static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
   {
@@ -1463,6 +1465,11 @@ static inline void split_page_memcg(struct page *head, int old_order, int new_or
   {
   }
  +static inline void folio_split_memcg_refs(struct folio *folio,
+		unsigned old_order, unsigned new_order)
+{
+}
+
   static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
   {
   	return 0;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 14b1963898a7..3e5ecc8f3d13 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3394,6 +3394,9 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
   			folio_set_young(new_folio);
   		if (folio_test_idle(folio))
   			folio_set_idle(new_folio);
+#ifdef CONFIG_MEMCG
+		new_folio->memcg_data = folio->memcg_data;
+#endif
    		folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio));
   	}
@@ -3525,18 +3528,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
   			}
   		}
  -		/*
-		 * Reset any memcg data overlay in the tail pages.
-		 * folio_nr_pages() is unreliable until prep_compound_page()
-		 * was called again.
-		 */
-#ifdef NR_PAGES_IN_LARGE_FOLIO
-		folio->_nr_pages = 0;
-#endif


I remember that we could trigger a warning without that, but I don't immediately find where that warning was. IIRC, if we'd split to order-0, page[1] would have indicated that it had a memcg set, and something bailed out.

Maybe Zi Yan recalls where that check fired.

The error I encountered is different. When I rebase my folio_split()
on top of David’s mapcount patchset, my original patch used folio_nr_pages()
after memcg split. Since memcg overlays with _nr_pages, when splitting
to order-0, folio->_nr_page is overwritten with memcg_data, causing
folio_nr_pages() to return a bogus value. With Matthew’s this patch,
memcg_data of page[1] is written inside __split_folio_to_order(),
so in theory __split_folio_to_order() can call folio_nr_pages() like
my original patch.

Ah, my memory comes back. Right, I fixed that by passing "old_order" instead when rebasing on your series and ...


For folio->_nr_pages = 0, I suppose it is trying to suppress any
page[1]->memcg_data != NULL check in the following code. But I could
not find any.

... that was likely an issue I ran into before rebasing on your code, where I had to effectively clear page[1]->memcg_data.

So all good, clearing folio->_nr_pages can be dropped.

--
Cheers,

David / dhildenb





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