On Mon, 20 May 2013 11:26:07 -0500 Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx> wrote:
> zswap is a thin backend for frontswap that takes pages that are in the process
> of being swapped out and attempts to compress them and store them in a
> RAM-based memory pool. This can result in a significant I/O reduction on the
> swap device and, in the case where decompressing from RAM is faster than
> reading from the swap device, can also improve workload performance.
>
> It also has support for evicting swap pages that are currently compressed in
> zswap to the swap device on an LRU(ish) basis. This functionality makes zswap a
> true cache in that, once the cache is full, the oldest pages can be moved out
> of zswap to the swap device so newer pages can be compressed and stored in
> zswap.
>
> This patch adds the zswap driver to mm/
>
> ...
Some random doodlings:
> +/*********************************
> +* zswap entry functions
> +**********************************/
> +#define ZSWAP_KMEM_CACHE_NAME "zswap_entry_cache"
I don't think this macro needs to exist - it is only used once.
> +static struct kmem_cache *zswap_entry_cache;
> +
> +static inline int zswap_entry_cache_create(void)
> +{
> + zswap_entry_cache =
> + kmem_cache_create(ZSWAP_KMEM_CACHE_NAME,
> + sizeof(struct zswap_entry), 0, 0, NULL);
Could use the KMEM_CACHE() helper here?
> + return (zswap_entry_cache == NULL);
> +}
> +
> +static inline void zswap_entry_cache_destory(void)
> +{
> + kmem_cache_destroy(zswap_entry_cache);
> +}
> +
> +static inline struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
> +{
> + struct zswap_entry *entry;
> + entry = kmem_cache_alloc(zswap_entry_cache, gfp);
> + if (!entry)
> + return NULL;
> + entry->refcount = 1;
> + return entry;
> +}
> +
> +static inline void zswap_entry_cache_free(struct zswap_entry *entry)
> +{
> + kmem_cache_free(zswap_entry_cache, entry);
> +}
> +
> +/* caller must hold the tree lock */
> +static inline void zswap_entry_get(struct zswap_entry *entry)
> +{
> + entry->refcount++;
> +}
> +
> +/* caller must hold the tree lock */
> +static inline int zswap_entry_put(struct zswap_entry *entry)
> +{
> + entry->refcount--;
> + return entry->refcount;
> +}
Don't bother with the explicit "inline". The compiler will ignore it
and will generally DTRT anyway.
> +/*********************************
> +* rbtree functions
> +**********************************/
> +static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
> +{
> + struct rb_node *node = root->rb_node;
> + struct zswap_entry *entry;
> +
> + while (node) {
> + entry = rb_entry(node, struct zswap_entry, rbnode);
> + if (entry->offset > offset)
> + node = node->rb_left;
> + else if (entry->offset < offset)
> + node = node->rb_right;
> + else
> + return entry;
> + }
> + return NULL;
> +}
> +
> +/*
> + * In the case that a entry with the same offset is found, it a pointer to
> + * the existing entry is stored in dupentry and the function returns -EEXIST
"it a pointer"?
> +*/
Missing leading space.
> +static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
> + struct zswap_entry **dupentry)
> +{
> + struct rb_node **link = &root->rb_node, *parent = NULL;
> + struct zswap_entry *myentry;
> +
> + while (*link) {
> + parent = *link;
> + myentry = rb_entry(parent, struct zswap_entry, rbnode);
> + if (myentry->offset > entry->offset)
> + link = &(*link)->rb_left;
> + else if (myentry->offset < entry->offset)
> + link = &(*link)->rb_right;
> + else {
> + *dupentry = myentry;
> + return -EEXIST;
> + }
> + }
> + rb_link_node(&entry->rbnode, parent, link);
> + rb_insert_color(&entry->rbnode, root);
> + return 0;
> +}
> +
>
> ...
>
> +/*********************************
> +* helpers
> +**********************************/
> +static inline bool zswap_is_full(void)
> +{
> + return (totalram_pages * zswap_max_pool_percent / 100 <
> + zswap_pool_pages);
> +}
We have had issues in the past where percentage-based tunables were too
coarse on very large machines. For example, a terabyte machine where 0
bytes is too small and 10GB is too large.
>
> ...
>
> +/*
> + * Attempts to free and entry by adding a page to the swap cache,
a/and/an/
> + * decompressing the entry data into the page, and issuing a
> + * bio write to write the page back to the swap device.
> + *
> + * This can be thought of as a "resumed writeback" of the page
> + * to the swap device. We are basically resuming the same swap
> + * writeback path that was intercepted with the frontswap_store()
> + * in the first place. After the page has been decompressed into
> + * the swap cache, the compressed version stored by zswap can be
> + * freed.
> + */
>
> ...
>
> +static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> + struct page *page)
> +{
> + struct zswap_tree *tree = zswap_trees[type];
> + struct zswap_entry *entry;
> + u8 *src, *dst;
> + unsigned int dlen;
> + int refcount, ret;
> +
> + /* find */
> + spin_lock(&tree->lock);
> + entry = zswap_rb_search(&tree->rbroot, offset);
> + if (!entry) {
> + /* entry was written back */
> + spin_unlock(&tree->lock);
> + return -1;
> + }
> + zswap_entry_get(entry);
> + spin_unlock(&tree->lock);
> +
> + /* decompress */
> + dlen = PAGE_SIZE;
> + src = (u8 *)zbud_map(tree->pool, entry->handle) +
> + sizeof(struct zswap_header);
> + dst = kmap_atomic(page);
> + ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src, entry->length,
> + dst, &dlen);
In all these places where the CPU alters the kmapped page: do we have
(or need) the appropriate cache flushing primitives?
flush_dcache_page() and similar.
> + kunmap_atomic(dst);
> + zbud_unmap(tree->pool, entry->handle);
> + BUG_ON(ret);
> +
> + spin_lock(&tree->lock);
> + refcount = zswap_entry_put(entry);
> + if (likely(refcount)) {
> + spin_unlock(&tree->lock);
> + return 0;
> + }
> + spin_unlock(&tree->lock);
> +
> + /*
> + * We don't have to unlink from the rbtree because
> + * zswap_writeback_entry() or zswap_frontswap_invalidate page()
> + * has already done this for us if we are the last reference.
> + */
> + /* free */
> +
> + zswap_free_entry(tree, entry);
> +
> + return 0;
> +}
> +
> +/* invalidates a single page */
"invalidate" is a very vague term in Linux. More specificity about
what actually happens to this page would be useful.
>
> ...
>
> +static struct zbud_ops zswap_zbud_ops = {
> + .evict = zswap_writeback_entry
> +};
> +
> +/* NOTE: this is called in atomic context from swapon and must not sleep */
Actually from frontswap, and calling a subsystem's ->init handler in
atomic context is quite lame - *of course* that handler will want to
allocate memory!
Whereabouts is the offending calling code and how do we fix it?
> +static void zswap_frontswap_init(unsigned type)
> +{
> + struct zswap_tree *tree;
> +
> + tree = kzalloc(sizeof(struct zswap_tree), GFP_ATOMIC);
> + if (!tree)
> + goto err;
> + tree->pool = zbud_create_pool(GFP_NOWAIT, &zswap_zbud_ops);
> + if (!tree->pool)
> + goto freetree;
> + tree->rbroot = RB_ROOT;
> + spin_lock_init(&tree->lock);
> + zswap_trees[type] = tree;
> + return;
> +
> +freetree:
> + kfree(tree);
> +err:
> + pr_err("alloc failed, zswap disabled for swap type %d\n", type);
> +}
> +
>
> ...
>
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