[RFC] Kill THP deferred split queue?

[Yang Shi <shy828301@xxxxxxxxx>]

Hi folks,

The THP deferred split queue is used to store PTE mapped THP (i.e.
partial unmapped THP) then they will get split by deferred split
shrinker when memory pressure kicks in.

Now the page reclaim could handle such cases nicely without calling
the shrinker. Since the THPs on deferred split queue is not PMD mapped
so they will be split unconditionally, then the unmapped sub pages
would get freed. Please see the below code snippet:

                             if (PageTransHuge(page)) {
                                        /* cannot split THP, skip it */
                                        if (!can_split_huge_page(page, NULL))
                                                goto activate_locked;
                                        /*
                                         * Split pages without a PMD map right
                                         * away. Chances are some or all of the
                                         * tail pages can be freed without IO.
                                         */
                                        if (!compound_mapcount(page) &&
                                            split_huge_page_to_list(page,
                                                                    page_list))
                                                goto activate_locked;
                                }

Then the unmapped pages will be moved to free_list by
move_pages_to_lru() called by shrink_inactive_list(). The mapped sub
pages will be kept on LRU. So, it does exactly the same thing as
deferred split shrinker and at the exact same timing.

The only benefit of shrinker is they can be split and freed via "echo
2 > /proc/sys/vm/drop_caches”, but I'm not sure how many people rely
on this?

The benefit of killing deferred split queue is code simplification.

Any comment is welcome.

Thanks,
Yang