Re: [PATCH v3 1/7] mm: zswap: add pool shrinking mechanism

[Yosry Ahmed]

On Sat, Jun 17, 2023 at 9:39 PM Sergey Senozhatsky
<senozhatsky@xxxxxxxxxxxx> wrote:
>
> On (23/06/12 11:38), Domenico Cerasuolo wrote:
> > +static int zswap_reclaim_entry(struct zswap_pool *pool)
> > +{
> > +     struct zswap_header *zhdr;
> > +     struct zswap_entry *entry;
> > +     struct zswap_tree *tree;
> > +     pgoff_t swpoffset;
> > +     int ret;
> > +
> > +     /* Get an entry off the LRU */
> > +     spin_lock(&pool->lru_lock);
> > +     if (list_empty(&pool->lru)) {
> > +             spin_unlock(&pool->lru_lock);
> > +             return -EINVAL;
> > +     }
> > +     entry = list_last_entry(&pool->lru, struct zswap_entry, lru);
> > +     list_del_init(&entry->lru);
>
> A quick question: should we zswap_entry_get() here?

We need to hold the tree lock for that, and the lock ordering is tree
lock -> lru lock. If we try to grab the tree lock here we may
deadlock. In the next code block we release the lru_lock, hold the
tree lock, make sure the entry is still valid in the tree, then do
zswap_entry_get().