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