Re: [PATCH] mm/list_lru: drain before clearing xarray entry on reparent

[Shakeel Butt <shakeel.butt@linux.dev>]

Hi Muchun, thanks for taking a look.

On Mon, Jun 01, 2026 at 05:54:01PM +0800, Muchun Song wrote:
> 
> 
> > On Jun 1, 2026, at 14:34, Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > 
> > memcg_reparent_list_lrus() clears the dying memcg's xarray entry with
> > xas_store(&xas, NULL) before reparenting its per-node lists into the
> > parent. This opens a window where a concurrent list_lru_del() arriving
> > for the dying memcg sees xa_load() == NULL, walks to the parent in
> > lock_list_lru_of_memcg(), takes the parent's per-node lock, and calls
> > list_del_init() on an item still physically linked on the dying
> > memcg's list.
> > 
> > If another in-flight thread holds the dying memcg's per-node lock at
> > the same moment (another list_lru_del, or a list_lru_walk_one running
> > an isolate callback), both threads modify ->next/->prev pointers on the
> > same physical list under different locks. Adjacent items can corrupt
> > each other's links.
> > 
> > Fix it by reversing the order: reparent each per-node list and mark the
> > child's list lru dead and then clear the xarray entry. Any concurrent
> > list_lru op that finds the still-set xarray entry either takes the dying
> > memcg's per-node lock (synchronizing with the drain) or sees LONG_MIN
> > and walks to the parent, where the items now live.
> > 
> > Fixes: fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope")
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > Reported-by: Chris Mason <clm@fb.com>
> > ---
> > mm/list_lru.c | 20 +++++++++-----------
> > 1 file changed, 9 insertions(+), 11 deletions(-)
> > 
> > diff --git a/mm/list_lru.c b/mm/list_lru.c
> > index dd29bcf8eb5f..ae55a52307db 100644
> > --- a/mm/list_lru.c
> > +++ b/mm/list_lru.c
> > @@ -473,26 +473,24 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren
> > 	mutex_lock(&list_lrus_mutex);
> > 	list_for_each_entry(lru, &memcg_list_lrus, list) {
> > 		struct list_lru_memcg *mlru;
> > - 		XA_STATE(xas, &lru->xa, memcg->kmemcg_id);
> > 
> > - 		/*
> > -		 * Lock the Xarray to ensure no on going list_lru_memcg
> > -		 * allocation and further allocation will see css_is_dying().
> > -		 */
> > - 		xas_lock_irq(&xas);
> > - 		mlru = xas_store(&xas, NULL);
> > - 		xas_unlock_irq(&xas);
> > + 		mlru = xa_load(&lru->xa, memcg->kmemcg_id);
> > 		if (!mlru)
> > 			continue;
> 
> Is it possible that concurrent threads running memcg_list_lru_alloc() could
> allocate a new mlru after this check passes? This could happen because the
> threads haven't noticed css_is_dying() yet. We would consequently miss the
> reparent operation for this list. So xas_lock_irq is necessary to serialize
> CSS_DYING setting here. Right?

Good question and it seems like Sashiko [1] raised a similar concern. However
please note that memcg_list_lru_alloc() uses CSS_DYING when it allocate a new
mlru but memcg_reparent_list_lrus() is called from offlice_css() callback and
the given css should already have CSS_DYING before calling offline_css(). There
is a rcu grace period between setting CSS_DYING and calling offline_css().

[1] https://sashiko.dev/#/patchset/20260601063408.2879011-1-shakeel.butt%40linux.dev

> 
> Thanks.
> Muchun
> 
> > 
> > 		/*
> > -		 * With Xarray value set to NULL, holding the lru lock below
> > -		 * prevents list_lru_{add,del,isolate} from touching the lru,
> > -		 * safe to reparent.
> > +		 * Reparent each per-node list and mark the child dead
> > +		 * (LONG_MIN) before clearing xarray entry otherwisw a
> > +		 * concurrent list_lru_del() may corrupt the list if it arrives
> > +		 * after xarray clear but before reparenting as
> > +		 * lock_list_lru_of_memcg will acquire parent's lock while the
> > +		 * item is still on child's list.
> > 		 */
> > 		for_each_node(i)
> > 			memcg_reparent_list_lru_one(lru, i, &mlru->node[i], parent);
> > 
> > + 		xa_erase(&lru->xa, memcg->kmemcg_id);

This one is more tricky. Sashiko said:

" Is it safe to use xa_erase() here instead of xa_erase_irq()?

The list_lru xarray is initialized with XA_FLAGS_LOCK_IRQ, and elements are
added holding the lock via xas_lock_irqsave(), which establishes an IRQ-safe
lock class.

Since xa_erase() internally calls spin_lock() without disabling local
interrupts, an interrupt firing while the lock is held could attempt to
re-acquire the same lock in __memcg_list_lru_alloc(), leading to a deadlock.

This could also trigger a lockdep warning for an inconsistent lock state. "

Initially I though this is a false positive as I couldn't find irq callers for
kmem_cache_alloc_lru() but then claude came up with more concrete scenario which
is below:

"""
For the shadow_nodes lru this lock is also acquired nested under the page
cache i_pages lock, which is irq-safe.  Adding a folio holds i_pages and
then allocates an xarray node through the shadow_nodes lru:

__filemap_add_folio()
  mapping_set_update(&xas, mapping)     // xas->xa_lru = &shadow_nodes
  xas_lock_irq(&xas)                    // holds mapping->i_pages
  xas_store() -> xas_alloc()
    kmem_cache_alloc_lru(radix_tree_node_cachep, xas->xa_lru, gfp)
      memcg_list_lru_alloc(memcg, &shadow_nodes, gfp)
        xas_lock_irqsave(&shadow_nodes->xa)   // shadow_nodes->xa under i_pages

and i_pages is taken from writeback completion in irq context:

__folio_end_writeback()
        xa_lock_irqsave(&mapping->i_pages, flags);

So with xa_erase() taking shadow_nodes->xa with irqs enabled:

CPU0 memcg_reparent_list_lrus()    CPU1 __filemap_add_folio()
  xa_erase(&shadow_nodes->xa)
    xa_lock(&shadow_nodes->xa)
                                   xas_lock_irq(&i_pages)  // holds i_pages
                                   ... memcg_list_lru_alloc()
                                     xas_lock_irqsave(&shadow_nodes->xa) // waits
  <io completion irq on CPU0>
  __folio_end_writeback()
    xa_lock_irqsave(&i_pages)  // waits

Can this deadlock, and should this be xa_erase_irq() to keep the irq-safe
acquisition that the removed xas_lock_irq() had?
"""

This seems more plausible and I think simply using xa_erase_irq() is more safe.

I will send a v2 with this change.