[PATCH v6 6/9] mm: multigenerational lru: aging

[Vlastimil Babka]

On 1/10/22 16:01, Michal Hocko wrote:
> On Thu 06-01-22 17:12:18, Michal Hocko wrote:
>> On Tue 04-01-22 13:22:25, Yu Zhao wrote:
>> > +static struct lru_gen_mm_walk *alloc_mm_walk(void)
>> > +{
>> > +	if (!current->reclaim_state || !current->reclaim_state->mm_walk)
>> > +		return kvzalloc(sizeof(struct lru_gen_mm_walk), GFP_KERNEL);
> 
> One thing I have overlooked completely. You cannot really use GFP_KERNEL
> allocation here because the reclaim context can be constrained (e.g.
> GFP_NOFS). This allocation will not do any reclaim as it is PF_MEMALLOC
> but I suspect that the lockdep will complain anyway.
> 
> Also kvmalloc is not really great here. a) vmalloc path is never
> executed for small objects and b) we do not really want to make a
> dependency between vmalloc and the reclaim (by vmalloc -> reclaim ->
> vmalloc).
> 
> Even if we rule out vmalloc and look at kmalloc alone. Is this really
> safe? I do not see any recursion prevention in the SL.B code. Maybe this
> just happens to work but the dependency should be really documented so
> that future SL.B changes won't break the whole scheme. 

Slab implementations drop all locks before calling into page allocator (thus
possibly reclaim) so slab itself should be fine and I don't expect it to
change. But we could eventually reach the page allocator recursively again,
that's true and not great.