Re: [PATCH v2 14/28] mm: mglru: prevent memory cgroup release in mglru

[Shakeel Butt]

On Wed, Dec 17, 2025 at 05:18:40PM -0500, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:38PM +0800, Qi Zheng wrote:
> > @@ -4242,6 +4244,13 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
> >  		}
> >  	}
> >  
> > +	rcu_read_lock();
> > +	memcg = folio_memcg(folio);
> > +	lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > +	max_seq = READ_ONCE((lruvec)->lrugen.max_seq);
> > +	gen = lru_gen_from_seq(max_seq);
> > +	mm_state = get_mm_state(lruvec);
> > +
> >  	arch_enter_lazy_mmu_mode();
> >  
> >  	pte -= (addr - start) / PAGE_SIZE;
> > @@ -4282,6 +4291,8 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
> >  	if (mm_state && suitable_to_scan(i, young))
> >  		update_bloom_filter(mm_state, max_seq, pvmw->pmd);
> >  
> > +	rcu_read_unlock();
> > +
> >  	return true;
> 
> This seems a bit long to be holding the rcu lock. Maybe do a get and a
> put instead?

This function is called under ptl lock, so at least for non-RT kernels,
the preemption is disabled across the function. Anyways no strong
opinion either way.