Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping()
From: David Hildenbrand (Red Hat)
Date: Fri Dec 19 2025 - 01:18:34 EST
On 12/19/25 05:12, Harry Yoo wrote:
On Thu, Dec 18, 2025 at 09:16:11PM +0800, Qi Zheng wrote:
On 12/18/25 9:04 PM, David Hildenbrand (Red Hat) wrote:
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.
Or simple use folio_memcg_begin(memcg)/folio_memcg_end(memcg) in all cases.
Or add a parameter to them:
struct mem_cgroup *folio_memcg_begin(struct folio *folio, bool get_refcnt)
{
struct mem_cgroup *memcg;
if (get_refcnt)
memcg = get_mem_cgroup_from_folio(folio);
else {
rcu_read_lock();
memcg = folio_memcg(folio);
}
return memcg;
}
void folio_memcg_end(struct mem_cgroup *memcg, bool get_refcnt)
{
if (get_refcnt)
mem_cgroup_put(memcg);
else
rcu_read_unlock();
}
I would like to vote for open coding as we do now, because I think hiding
the RCU lock / refcount acquisition into a less obvious API doesn't make
it more readable.
I wouldn't do it in an API as proposed above.
I prefer to not have magical RCU locking in every caller. Easy to get wrong.
See how we did something similar in the pte_*map*() vs. pte_unmap() API, without requiring all callers to open-code this.
--
Cheers
David