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    SubjectRe: [PATCH 2/8] Basic zcache functionality
    Nitin Gupta wrote:
    > +/*
    > + * Individual percpu values can go negative but the sum across all CPUs
    > + * must always be positive (we store various counts). So, return sum as
    > + * unsigned value.
    > + */
    > +static u64 zcache_get_stat(struct zcache_pool *zpool,
    > + enum zcache_pool_stats_index idx)
    > +{
    > + int cpu;
    > + s64 val = 0;
    > +
    > + for_each_possible_cpu(cpu) {
    > + unsigned int start;
    > + struct zcache_pool_stats_cpu *stats;
    > +
    > + stats = per_cpu_ptr(zpool->stats, cpu);
    > + do {
    > + start = u64_stats_fetch_begin(&stats->syncp);
    > + val += stats->count[idx];
    > + } while (u64_stats_fetch_retry(&stats->syncp, start));

    Can we use 'struct percpu_counter' for this? OTOH, the warning on top of
    include/linux/percpu_counter.h makes me think not.

    > + }
    > +
    > + BUG_ON(val < 0);

    BUG_ON() seems overly aggressive. How about

    if (WARN_ON(val < 0))
    return 0;

    > + return val;
    > +}
    > +
    > +static void zcache_add_stat(struct zcache_pool *zpool,
    > + enum zcache_pool_stats_index idx, s64 val)
    > +{
    > + struct zcache_pool_stats_cpu *stats;
    > +
    > + preempt_disable();
    > + stats = __this_cpu_ptr(zpool->stats);
    > + u64_stats_update_begin(&stats->syncp);
    > + stats->count[idx] += val;
    > + u64_stats_update_end(&stats->syncp);
    > + preempt_enable();

    What is the preempt_disable/preempt_enable trying to do here?

    > +static void zcache_destroy_pool(struct zcache_pool *zpool)
    > +{
    > + int i;
    > +
    > + if (!zpool)
    > + return;
    > +
    > + spin_lock(&zcache->pool_lock);
    > + zcache->num_pools--;
    > + for (i = 0; i < MAX_ZCACHE_POOLS; i++)
    > + if (zcache->pools[i] == zpool)
    > + break;
    > + zcache->pools[i] = NULL;
    > + spin_unlock(&zcache->pool_lock);
    > +
    > + if (!RB_EMPTY_ROOT(&zpool->inode_tree)) {

    Use WARN_ON here to get a stack trace?

    > + pr_warn("Memory leak detected. Freeing non-empty pool!\n");
    > + zcache_dump_stats(zpool);
    > + }
    > +
    > + free_percpu(zpool->stats);
    > + kfree(zpool);
    > +}
    > +
    > +/*
    > + * Allocate a new zcache pool and set default memlimit.
    > + *
    > + * Returns pool_id on success, negative error code otherwise.
    > + */
    > +int zcache_create_pool(void)
    > +{
    > + int ret;
    > + u64 memlimit;
    > + struct zcache_pool *zpool = NULL;
    > +
    > + spin_lock(&zcache->pool_lock);
    > + if (zcache->num_pools == MAX_ZCACHE_POOLS) {
    > + spin_unlock(&zcache->pool_lock);
    > + pr_info("Cannot create new pool (limit: %u)\n",
    > + MAX_ZCACHE_POOLS);
    > + ret = -EPERM;
    > + goto out;
    > + }
    > + zcache->num_pools++;
    > + spin_unlock(&zcache->pool_lock);
    > +
    > + zpool = kzalloc(sizeof(*zpool), GFP_KERNEL);
    > + if (!zpool) {
    > + spin_lock(&zcache->pool_lock);
    > + zcache->num_pools--;
    > + spin_unlock(&zcache->pool_lock);
    > + ret = -ENOMEM;
    > + goto out;
    > + }

    Why not kmalloc() an new struct zcache_pool object first and then take
    zcache->pool_lock() and check for MAX_ZCACHE_POOLS? That should make the
    locking little less confusing here.

    > +
    > + zpool->stats = alloc_percpu(struct zcache_pool_stats_cpu);
    > + if (!zpool->stats) {
    > + ret = -ENOMEM;
    > + goto out;
    > + }
    > +
    > + rwlock_init(&zpool->tree_lock);
    > + seqlock_init(&zpool->memlimit_lock);
    > + zpool->inode_tree = RB_ROOT;
    > +
    > + memlimit = zcache_pool_default_memlimit_perc_ram *
    > + ((totalram_pages << PAGE_SHIFT) / 100);
    > + memlimit &= PAGE_MASK;
    > + zcache_set_memlimit(zpool, memlimit);
    > +
    > + /* Add to pool list */
    > + spin_lock(&zcache->pool_lock);
    > + for (ret = 0; ret < MAX_ZCACHE_POOLS; ret++)
    > + if (!zcache->pools[ret])
    > + break;
    > + zcache->pools[ret] = zpool;
    > + spin_unlock(&zcache->pool_lock);
    > +
    > +out:
    > + if (ret < 0)
    > + zcache_destroy_pool(zpool);
    > +
    > + return ret;
    > +}

    > +/*
    > + * Allocate memory for storing the given page and insert
    > + * it in the given node's page tree at location 'index'.
    > + *
    > + * Returns 0 on success, negative error code on failure.
    > + */
    > +static int zcache_store_page(struct zcache_inode_rb *znode,
    > + pgoff_t index, struct page *page)
    > +{
    > + int ret;
    > + unsigned long flags;
    > + struct page *zpage;
    > + void *src_data, *dest_data;
    > +
    > + zpage = alloc_page(GFP_NOWAIT);
    > + if (!zpage) {
    > + ret = -ENOMEM;
    > + goto out;
    > + }
    > + zpage->index = index;
    > +
    > + src_data = kmap_atomic(page, KM_USER0);
    > + dest_data = kmap_atomic(zpage, KM_USER1);
    > + memcpy(dest_data, src_data, PAGE_SIZE);
    > + kunmap_atomic(src_data, KM_USER0);
    > + kunmap_atomic(dest_data, KM_USER1);

    copy_highpage()

    > +
    > + spin_lock_irqsave(&znode->tree_lock, flags);
    > + ret = radix_tree_insert(&znode->page_tree, index, zpage);
    > + spin_unlock_irqrestore(&znode->tree_lock, flags);
    > + if (unlikely(ret))
    > + __free_page(zpage);
    > +
    > +out:
    > + return ret;
    > +}

    > +/*
    > + * cleancache_ops.get_page
    > + *
    > + * Locates stored zcache page using <pool_id, inode_no, index>.
    > + * If found, copies it to the given output page 'page' and frees
    > + * zcache copy of the same.
    > + *
    > + * Returns 0 if requested page found, -1 otherwise.
    > + */
    > +static int zcache_get_page(int pool_id, ino_t inode_no,
    > + pgoff_t index, struct page *page)
    > +{
    > + int ret = -1;
    > + unsigned long flags;
    > + struct page *src_page;
    > + void *src_data, *dest_data;
    > + struct zcache_inode_rb *znode;
    > + struct zcache_pool *zpool = zcache->pools[pool_id];
    > +
    > + znode = zcache_find_inode(zpool, inode_no);
    > + if (!znode)
    > + goto out;
    > +
    > + BUG_ON(znode->inode_no != inode_no);

    Maybe use WARN_ON here and return -1?

    > +
    > + spin_lock_irqsave(&znode->tree_lock, flags);
    > + src_page = radix_tree_delete(&znode->page_tree, index);
    > + if (zcache_inode_is_empty(znode))
    > + zcache_inode_isolate(znode);
    > + spin_unlock_irqrestore(&znode->tree_lock, flags);
    > +
    > + kref_put(&znode->refcount, zcache_inode_release);
    > +
    > + if (!src_page)
    > + goto out;
    > +
    > + src_data = kmap_atomic(src_page, KM_USER0);
    > + dest_data = kmap_atomic(page, KM_USER1);
    > + memcpy(dest_data, src_data, PAGE_SIZE);
    > + kunmap_atomic(src_data, KM_USER0);
    > + kunmap_atomic(dest_data, KM_USER1);

    The above sequence can be replaced with copy_highpage().

    > +
    > + flush_dcache_page(page);
    > +
    > + __free_page(src_page);
    > +
    > + zcache_dec_stat(zpool, ZPOOL_STAT_PAGES_STORED);
    > + ret = 0; /* success */
    > +
    > +out:
    > + return ret;
    > +}


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