Re: [PATCH -mm -v7 9/9] mm, THP, swap: Delay splitting THP during swap out

["Huang\, Ying" <ying.huang@intel.com>]

Johannes Weiner <hannes@cmpxchg.org> writes:

> On Tue, Mar 28, 2017 at 01:32:09PM +0800, Huang, Ying wrote:
>> @@ -183,12 +184,53 @@ void __delete_from_swap_cache(struct page *page)
>>  	ADD_CACHE_INFO(del_total, nr);
>>  }
>>  
>> +#ifdef CONFIG_THP_SWAP_CLUSTER
>> +int add_to_swap_trans_huge(struct page *page, struct list_head *list)
>> +{
>> +	swp_entry_t entry;
>> +	int ret = 0;
>> +
>> +	/* cannot split, which may be needed during swap in, skip it */
>> +	if (!can_split_huge_page(page, NULL))
>> +		return -EBUSY;
>> +	/* fallback to split huge page firstly if no PMD map */
>> +	if (!compound_mapcount(page))
>> +		return 0;
>> +	entry = get_huge_swap_page();
>> +	if (!entry.val)
>> +		return 0;
>> +	if (mem_cgroup_try_charge_swap(page, entry, HPAGE_PMD_NR)) {
>> +		__swapcache_free(entry, true);
>> +		return -EOVERFLOW;
>> +	}
>> +	ret = add_to_swap_cache(page, entry,
>> +				__GFP_HIGH | __GFP_NOMEMALLOC|__GFP_NOWARN);
>> +	/* -ENOMEM radix-tree allocation failure */
>> +	if (ret) {
>> +		__swapcache_free(entry, true);
>> +		return 0;
>> +	}
>> +	ret = split_huge_page_to_list(page, list);
>> +	if (ret) {
>> +		delete_from_swap_cache(page);
>> +		return -EBUSY;
>> +	}
>> +	return 1;
>> +}
>> +#else
>> +static inline int add_to_swap_trans_huge(struct page *page,
>> +					 struct list_head *list)
>> +{
>> +	return 0;
>> +}
>> +#endif
>> +
>>  /**
>>   * add_to_swap - allocate swap space for a page
>>   * @page: page we want to move to swap
>>   *
>>   * Allocate swap space for the page and add the page to the
>> - * swap cache.  Caller needs to hold the page lock. 
>> + * swap cache.  Caller needs to hold the page lock.
>>   */
>>  int add_to_swap(struct page *page, struct list_head *list)
>>  {
>> @@ -198,6 +240,18 @@ int add_to_swap(struct page *page, struct list_head *list)
>>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>>  	VM_BUG_ON_PAGE(!PageUptodate(page), page);
>>  
>> +	if (unlikely(PageTransHuge(page))) {
>> +		err = add_to_swap_trans_huge(page, list);
>> +		switch (err) {
>> +		case 1:
>> +			return 1;
>> +		case 0:
>> +			/* fallback to split firstly if return 0 */
>> +			break;
>> +		default:
>> +			return 0;
>> +		}
>> +	}
>>  	entry = get_swap_page();
>>  	if (!entry.val)
>>  		return 0;
>
> add_to_swap_trans_huge() is too close a copy of add_to_swap(), which
> makes the code error prone for future modifications to the swap slot
> allocation protocol.
>
> This should read:
>
> retry:
> 	entry = get_swap_page(page);
> 	if (!entry.val) {
> 		if (PageTransHuge(page)) {
> 			split_huge_page_to_list(page, list);
> 			goto retry;
> 		}
> 		return 0;
> 	}

If the swap space is used up, that is, get_swap_page() cannot allocate
even 1 swap entry for a normal page.  We will split THP unnecessarily
with the change, but in the original code, we just skip the THP.  There
may be a performance regression here.  Similar problem exists for
mem_cgroup_try_charge_swap() too.  If the mem cgroup exceeds the swap
limit, the THP will be split unnecessary with the change too.

> And get_swap_page(), mem_cgroup_try_charge_swap() etc. should all
> check PageTransHuge() instead of having extra parameters or separate
> code paths for the huge page case.
>
> In general, don't try to tack this feature onto the side of the
> VM. Because right now, this looks a bit like the hugetlb code, with
> one big branch in the beginning that opens up an alternate
> reality. Instead, these functions should handle THP all the way down
> the stack, and without passing down redundant information.

Yes.  We should share the code as much as possible.  I just have some
questions as above.  Could you help me on that?

Best Regards,
Huang, Ying

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