Re: [PATCH -mm -v9 1/3] mm, THP, swap: Delay splitting THP during swap out

Huang, Ying
2017-04-20T00:43:18Z

Hi, Johannes,

Johannes Weiner <hannes@cmpxchg.org> writes:

> On Wed, Apr 19, 2017 at 03:06:23PM +0800, Huang, Ying wrote:
>> @@ -206,17 +212,34 @@ int add_to_swap(struct page *page, struct list_head *list)
>> */
>> err = add_to_swap_cache(page, entry,
>> __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
>> -
>> - if (!err) {
>> - return 1;
>> - } else { /* -ENOMEM radix-tree allocation failure */
>> + /* -ENOMEM radix-tree allocation failure */
>> + if (err)
>> /*
>> * add_to_swap_cache() doesn't return -EEXIST, so we can safely
>> * clear SWAP_HAS_CACHE flag.
>> */
>> - swapcache_free(entry);
>> - return 0;
>> + goto fail_free;
>> +
>> + if (unlikely(PageTransHuge(page))) {
>> + err = split_huge_page_to_list(page, list);
>> + if (err) {
>> + delete_from_swap_cache(page);
>> + return 0;
>> + }
>> }
>> +
>> + return 1;
>> +
>> +fail_free:
>> + if (unlikely(PageTransHuge(page)))
>> + swapcache_free_cluster(entry);
>> + else
>> + swapcache_free(entry);
>> +fail:
>> + if (unlikely(PageTransHuge(page)) &&
>> + !split_huge_page_to_list(page, list))
>> + goto retry;
>
> May I ask why you added the unlikelies there? Can you generally say
> THPs are unlikely in this path? Is the swap-out path so hot that
> branch layout is critical? I doubt either is true.

I just found there are unlikely() encloses PageTransHuge() in the
original add_to_swap(), so I just follow the original style. But I
don't think they make much sense too. Will remove them in the next
version.

> Also please mention changes like these in the changelog next time.

Sorry and will do that in the future.

Best Regards,
Huang, Ying