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 Thu, Mar 30, 2017 at 12:15:13PM +0800, Huang, Ying wrote:
>> Johannes Weiner <hannes@cmpxchg.org> writes:
>> > On Tue, Mar 28, 2017 at 01:32:09PM +0800, Huang, Ying wrote:
>> >> @@ -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.
>
> If we skip the page, we're swapping out another page hotter than this
> one. Giving THP preservation priority over LRU order is an issue best
> kept for a separate patch set;

In my original patch, if we failed to allocate the swap space for a THP,
and we can allocate the swap space for a normal page, we will split the
THP.  We skip the page only if we cannot allocate the swap space for a
normal page, that is, nr_swap_pages is 0.  So we will not give THP
preservation priority over LRU order in the patch.

> this one is supposed to be a mechanical
> implementation of THP swapping. Let's nail down the basics first.

Yes.  So I tried to keep the original behavior to deal with THP if we
cannot allocate the swap space (a swap cluster) for a whole THP.

Per my understanding, the difference between what you suggested and the
original behavior is that, when nr_swap_pages is 0, whether to split the
THP.

> Such a decision would need proof that splitting THPs on full swap
> devices is a concern for real applications. I would assume that we're
> pretty close to OOM anyway; it's much more likely that a single slot
> frees up than a full cluster, at which point we'll be splitting THPs
> anyway; etc. I have my doubts that this would be measurable.
>
> But even if so, I don't think we'd have to duplicate the main code
> flow to handle this corner case. You can extend get_swap_page() to
> return an error code that tells add_to_swap() whether to split and
> retry, or to fail and move on. So this way should be future proof.

Yes.  I will try to merge add_to_swap_trans_huge() into add_to_swap() in
the next version.  But if we want to keep the original behavior, we will
need an extra "nr_entries" parameter for mem_cgroup_try_charge_swap().

Best Regards,
Huang, Ying

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