Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping()
From: David Hildenbrand (Red Hat)
Date: Thu Dec 18 2025 - 08:05:06 EST
On 12/18/25 14:00, Qi Zheng wrote:
On 12/18/25 7:56 PM, David Hildenbrand (Red Hat) wrote:
On 12/18/25 12:40, Qi Zheng wrote:
On 12/18/25 5:43 PM, David Hildenbrand (Red Hat) wrote:
On 12/18/25 10:36, Qi Zheng wrote:
On 12/18/25 5:09 PM, David Hildenbrand (Red Hat) wrote:
On 12/17/25 08:27, Qi Zheng wrote:
From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
In the near future, a folio will no longer pin its corresponding
memory cgroup. To ensure safety, it will only be appropriate to
hold the rcu read lock or acquire a reference to the memory cgroup
returned by folio_memcg(), thereby preventing it from being released.
In the current patch, the rcu read lock is employed to safeguard
against the release of the memory cgroup in folio_migrate_mapping().
We usually avoid talking about "patches".
Got it.
In __folio_migrate_mapping(), the rcu read lock ...
Will do.
This serves as a preparatory measure for the reparenting of the
LRU pages.
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
---
mm/migrate.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/mm/migrate.c b/mm/migrate.c
index 5169f9717f606..8bcd588c083ca 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -671,6 +671,7 @@ static int __folio_migrate_mapping(struct
address_space *mapping,
struct lruvec *old_lruvec, *new_lruvec;
struct mem_cgroup *memcg;
+ rcu_read_lock();
memcg = folio_memcg(folio);
In general, LGTM
I wonder, though, whether we should embed that in the ABI.
Like "lock RCU and get the memcg" in one operation, to the "return
memcg
and unock rcu" in another operation.
Do you mean adding a helper function like get_mem_cgroup_from_folio()?
Right, something like
memcg = folio_memcg_begin(folio);
folio_memcg_end(memcg);
For some longer or might-sleep critical sections (such as those pointed
by Johannes), perhaps it can be defined like this:
struct mem_cgroup *folio_memcg_begin(struct folio *folio)
{
return get_mem_cgroup_from_folio(folio);
}
void folio_memcg_end(struct mem_cgroup *memcg)
{
mem_cgroup_put(memcg);
}
But for some short critical sections, using RCU lock directly might
be the most convention option?
Then put the rcu read locking in there instead?
So for some longer or might-sleep critical sections, using:
memcg = folio_memcg_begin(folio);
do_some_thing(memcg);
folio_memcg_end(folio);
for some short critical sections, using:
rcu_read_lock();
memcg = folio_memcg(folio);
do_some_thing(memcg);
rcu_read_unlock();
Right?
What I mean is:
memcg = folio_memcg_begin(folio);
do_some_thing(memcg);
folio_memcg_end(folio);
but do the rcu_read_lock() in folio_memcg_begin() and the rcu_read_unlock() in folio_memcg_end().
You could also have (expensive) variants, as you describe, that mess with getting/dopping the memcg.
But my points was about hiding the rcu details in a set of helpers.
Sorry if what I say is confusing.
--
Cheers
David