Re: [RFC PATCH v2 1/7] mm: zswap: add pool shrinking mechanism

Johannes Weiner
2023-06-08T17:05:08Z

On Thu, Jun 08, 2023 at 12:52:51PM -0400, Johannes Weiner wrote:
> On Tue, Jun 06, 2023 at 04:56:05PM +0200, Domenico Cerasuolo wrote:
> > @@ -584,14 +601,70 @@ static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
> > return NULL;
> > }
> >
> > +static int zswap_shrink(struct zswap_pool *pool)
> > +{
> > + struct zswap_entry *lru_entry, *tree_entry = NULL;
> > + struct zswap_header *zhdr;
> > + struct zswap_tree *tree;
> > + int swpoffset;
> > + int ret;
> > +
> > + /* get a reclaimable entry from LRU */
> > + spin_lock(&pool->lru_lock);
> > + if (list_empty(&pool->lru)) {
> > + spin_unlock(&pool->lru_lock);
> > + return -EINVAL;
> > + }
> > + lru_entry = list_last_entry(&pool->lru, struct zswap_entry, lru);
> > + list_del_init(&lru_entry->lru);
> > + zhdr = zpool_map_handle(pool->zpool, lru_entry->handle, ZPOOL_MM_RO);
> > + tree = zswap_trees[swp_type(zhdr->swpentry)];
> > + zpool_unmap_handle(pool->zpool, lru_entry->handle);
> > + /*
> > + * Once the pool lock is dropped, the lru_entry might get freed. The
> > + * swpoffset is copied to the stack, and lru_entry isn't deref'd again
> > + * until the entry is verified to still be alive in the tree.
> > + */
> > + swpoffset = swp_offset(zhdr->swpentry);
> > + spin_unlock(&pool->lru_lock);
> > +
> > + /* hold a reference from tree so it won't be freed during writeback */
> > + spin_lock(&tree->lock);
> > + tree_entry = zswap_entry_find_get(&tree->rbroot, swpoffset);
> > + if (tree_entry != lru_entry) {
> > + if (tree_entry)
> > + zswap_entry_put(tree, tree_entry);
> > + spin_unlock(&tree->lock);
> > + return -EAGAIN;
> > + }
> > + spin_unlock(&tree->lock);
> > +
> > + ret = zswap_writeback_entry(pool->zpool, lru_entry->handle);
> > +
> > + spin_lock(&tree->lock);
> > + if (ret) {
> > + spin_lock(&pool->lru_lock);
> > + list_move(&lru_entry->lru, &pool->lru);
> > + spin_unlock(&pool->lru_lock);
> > + }
> > + zswap_entry_put(tree, tree_entry);
>
> On re-reading this, I find the lru_entry vs tree_entry distinction
> unnecessarily complicated. Once it's known that the thing coming off
> the LRU is the same thing as in the tree, there is only "the entry".
>
> How about 'entry' and 'tree_entry', and after validation use 'entry'
> throughout the rest of the function?

Even better, safe the tree_entry entirely by getting the reference
from the LRU already, and then just search the tree for a match:

/* Get an entry off the LRU */
spin_lock(&pool->lru_lock);
entry = list_last_entry();
list_del(&entry->lru);
zswap_entry_get(entry);
spin_unlock(&pool->lru_lock);

/* Check for invalidate() race */
spin_lock(&tree->lock);
if (entry != zswap_rb_search(&tree->rbroot, swpoffset)) {
ret = -EAGAIN;
goto put_unlock;
}
spin_unlock(&tree->lock);

ret = zswap_writeback_entry();

spin_lock(&tree->lock);
if (ret) {
put_back_on_lru();
goto put_unlock;
}

/* Check for invalidate() race */
if (entry != zswap_rb_search(&tree->rbroot, swpoffset))
goto put_unlock;

/* Drop base reference */
zswap_entry_put(tree, entry);

put_unlock:
/* Drop local reference */
zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);

return ret ? -EAGAIN : 0;