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SubjectRe: [Update][PATCH v5 7/9] mm/swap: Add cache for swap slots allocation
On Tue 17-01-17 10:55:47, Huang, Ying wrote:
[...]
> +int free_swap_slot(swp_entry_t entry)
> +{
> + struct swap_slots_cache *cache;
> +
> + BUG_ON(!swap_slot_cache_initialized);
> +
> + cache = &get_cpu_var(swp_slots);
> + if (use_swap_slot_cache && cache->slots_ret) {
> + spin_lock_irq(&cache->free_lock);
> + /* Swap slots cache may be deactivated before acquiring lock */
> + if (!use_swap_slot_cache) {
> + spin_unlock_irq(&cache->free_lock);
> + goto direct_free;
> + }
> + if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE) {
> + /*
> + * Return slots to global pool.
> + * The current swap_map value is SWAP_HAS_CACHE.
> + * Set it to 0 to indicate it is available for
> + * allocation in global pool
> + */
> + swapcache_free_entries(cache->slots_ret, cache->n_ret);
> + cache->n_ret = 0;
> + }
> + cache->slots_ret[cache->n_ret++] = entry;
> + spin_unlock_irq(&cache->free_lock);
> + } else {
> +direct_free:
> + swapcache_free_entries(&entry, 1);
> + }
> + put_cpu_var(swp_slots);
> +
> + return 0;
> +}
> +
> +swp_entry_t get_swap_page(void)
> +{
> + swp_entry_t entry, *pentry;
> + struct swap_slots_cache *cache;
> +
> + /*
> + * Preemption need to be turned on here, because we may sleep
> + * in refill_swap_slots_cache(). But it is safe, because
> + * accesses to the per-CPU data structure are protected by a
> + * mutex.
> + */

the comment doesn't really explain why it is safe. THere are other users
which are not using the lock. E.g. just look at free_swap_slot above.
How can
cache->slots_ret[cache->n_ret++] = entry;
be safe wrt.
pentry = &cache->slots[cache->cur++];
entry = *pentry;

Both of them might touch the same slot, no? Btw. I would rather prefer
this would be a follow up fix with the trace and the detailed
explanation.

> + cache = raw_cpu_ptr(&swp_slots);
> +
> + entry.val = 0;
> + if (check_cache_active()) {
> + mutex_lock(&cache->alloc_lock);
> + if (cache->slots) {
> +repeat:
> + if (cache->nr) {
> + pentry = &cache->slots[cache->cur++];
> + entry = *pentry;
> + pentry->val = 0;
> + cache->nr--;
> + } else {
> + if (refill_swap_slots_cache(cache))
> + goto repeat;
> + }
> + }
> + mutex_unlock(&cache->alloc_lock);
> + if (entry.val)
> + return entry;
> + }
> +
> + get_swap_pages(1, &entry);
> +
> + return entry;
> +}
--
Michal Hocko
SUSE Labs

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