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From: Pekka Enberg on 18 Jul 2010 04:20
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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From: Pekka Enberg on 18 Jul 2010 04:30
Nitin Gupta wrote:
> +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();
> +
> +}

You should probably use this_cpu_inc() here.
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From: Eric Dumazet on 18 Jul 2010 04:50
Le vendredi 16 juillet 2010 à 18:07 +0530, Nitin Gupta a écrit :

> This particular patch implemets basic functionality only:
> +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));
> + }
> +
> + BUG_ON(val < 0);
> + return val;
> +}

Sorry this is wrong.

Inside the fetch/retry block you should not do the addition to val, only
a read of value to a temporary variable, since this might be done
several times.

You want something like :

static u64 zcache_get_stat(struct zcache_pool *zpool,
enum zcache_pool_stats_index idx)
{
int cpu;
s64 temp, 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);
temp = stats->count[idx];
} while (u64_stats_fetch_retry(&stats->syncp, start));
val += temp;
}

....
}



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From: Nitin Gupta on 18 Jul 2010 05:50

On 07/18/2010 01:44 PM, Pekka Enberg wrote:
> 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.
>

Yes, that warning only scared me :)


>> + }
>> +
>> + BUG_ON(val < 0);
>
> BUG_ON() seems overly aggressive. How about
>
> if (WARN_ON(val < 0))
> return 0;
>

Yes, this sounds better. I will change it.


>> + 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?
>

On 32-bit there will be no seqlock to protect this value. So, if we
get preempted after __this_cpu_ptr(), two CPUs can end up racy-writing
to the same variable. I think for the same reason this_cpu_add() finally
does increment with preempt disabled.

Also, I think we shouldn't use this_cpu_add (as you suggested in
another mail) since we have to do this_cpu_ptr() first to get access
to seqlock (stats->syncp) anyways. So, simple increment on thus
obtained pcpu pointer should be okay.


>> +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?
>

This sounds better, will change it.


>> + 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.
>

kmalloc() before this check should be better. This also avoids unnecessary
num_pools decrement later if kmalloc fails.


>> +
>> + 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()
>

Ok. But we will again have to open-code this memcpy() when we start using
xvmalloc (patch 7/8). Same applies to another instance you pointed out.


>> +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?
>

okay.


Thanks for the review.
Nitin
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From: Nitin Gupta on 18 Jul 2010 06:00
On 07/18/2010 02:14 PM, Eric Dumazet wrote:
> Le vendredi 16 juillet 2010 à 18:07 +0530, Nitin Gupta a écrit :
>
>> This particular patch implemets basic functionality only:
>> +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));
>> + }
>> +
>> + BUG_ON(val < 0);
>> + return val;
>> +}
>
> Sorry this is wrong.
>
> Inside the fetch/retry block you should not do the addition to val, only
> a read of value to a temporary variable, since this might be done
> several times.
>
> You want something like :
>
> static u64 zcache_get_stat(struct zcache_pool *zpool,
> enum zcache_pool_stats_index idx)
> {
> int cpu;
> s64 temp, 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);
> temp = stats->count[idx];
> } while (u64_stats_fetch_retry(&stats->syncp, start));
> val += temp;
> }
>
> ...
> }
>
>

Oh, my bad. Thanks for the fix!

On a side note: u64_stats_* should probably be renamed to stats64_* since
they are equally applicable for s64.


Thanks,
Nitin
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