Re: [PATCH v7 3/6] zswap: make shrinking memcg-aware

On Wed, Nov 29, 2023 at 7:17 AM Johannes Weiner hannes@cmpxchg.org wrote:

On Mon, Nov 27, 2023 at 03:45:57PM -0800, Nhat Pham wrote:

...

static void shrink_worker(struct work_struct *w) { struct zswap_pool *pool = container_of(w, typeof(*pool), shrink_work);

• struct mem_cgroup *memcg;
  int ret, failures = 0;
  
  

• /* global reclaim will select cgroup in a round-robin fashion. */
  do {
  

•         ret = zswap_reclaim_entry(pool);
  

•         if (ret) {
  

•                 zswap_reject_reclaim_fail++;
  

•                 if (ret != -EAGAIN)
  

•                         break;
  

•         spin_lock(&zswap_pools_lock);
  

•         memcg = pool->next_shrink =
  

•                 mem_cgroup_iter_online(NULL, pool->next_shrink, NULL, true);
  

•         /* full round trip */
  

•         if (!memcg) {
  

•                 spin_unlock(&zswap_pools_lock);
                  if (++failures == MAX_RECLAIM_RETRIES)
                          break;
  

•                 goto resched;
          }
  

•         /*
  

•          * Acquire an extra reference to the iterated memcg in case the
  

•          * original reference is dropped by the zswap offlining callback.
  

•          */
  

•         css_get(&memcg->css);
  

struct mem_cgroup isn't defined when !CONFIG_MEMCG. This needs a mem_cgroup_get() wrapper and a dummy function for no-memcg builds.

I got this exact same issue a couple of versions ago, but it was hidden behind another helper function which can be implemented as a no-op in the case of !CONFIG_MEMCG, so I forgot about it until now. It always strikes me a bit weird that we have mem_cgroup_put() but not an equivalent get - let me correct that.

With that fixed, though, everything else looks good to me:

Acked-by: Johannes Weiner hannes@cmpxchg.org

Thanks for the review, Johannes!