thp: split_huge_page paging

Paging logic that splits the page before it is unmapped and added to swap
to ensure backwards compatibility with the legacy swap code.  Eventually
swap should natively pageout the hugepages to increase performance and
decrease seeking and fragmentation of swap space.  swapoff can just skip
over huge pmd as they cannot be part of swap yet.  In add_to_swap be
careful to split the page only if we got a valid swap entry so we don't
split hugepages with a full swap.

In theory we could split pages before isolating them during the lru scan,
but for khugepaged to be safe, I'm relying on either mmap_sem write mode,
or PG_lock taken, so split_huge_page has to run either with mmap_sem
read/write mode or PG_lock taken.  Calling it from isolate_lru_page would
make locking more complicated, in addition to that split_huge_page would
deadlock if called by __isolate_lru_page because it has to take the lru
lock to add the tail pages.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 11 insertions, 0 deletions

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 2323a8039a98..6a283cc9317c 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -386,6 +386,8 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
         struct task_struct *tsk;
         struct anon_vma *av;
 
+        if (unlikely(split_huge_page(page)))
+                return;
         read_lock(&tasklist_lock);
         av = page_lock_anon_vma(page);
         if (av == NULL) /* Not actually mapped anymore */
diff --git a/mm/rmap.c b/mm/rmap.c
index c95d2ba27a0b..a3197a8a295b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1400,6 +1400,7 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
         int ret;
 
         BUG_ON(!PageLocked(page));
+        BUG_ON(PageTransHuge(page));
 
         if (unlikely(PageKsm(page)))
                 ret = try_to_unmap_ksm(page, flags);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index e10f5833167f..5c8cfabbc9bc 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -157,6 +157,12 @@ int add_to_swap(struct page *page)
         if (!entry.val)
                 return 0;
 
+        if (unlikely(PageTransHuge(page)))
+                if (unlikely(split_huge_page(page))) {
+                        swapcache_free(entry, NULL);
+                        return 0;
+                }
+
         /*
          * Radix-tree node allocations from PF_MEMALLOC contexts could
          * completely exhaust the page allocator. __GFP_NOMEMALLOC
diff --git a/mm/swapfile.c b/mm/swapfile.c
index b6adcfbf6f48..07a458d72fa8 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -964,6 +964,8 @@ static inline int unuse_pmd_range(struct vm_area_struct *vma, pud_t *pud,
         pmd = pmd_offset(pud, addr);
         do {
                 next = pmd_addr_end(addr, end);
+                if (unlikely(pmd_trans_huge(*pmd)))
+                        continue;
                 if (pmd_none_or_clear_bad(pmd))
                         continue;
                 ret = unuse_pte_range(vma, pmd, addr, next, entry, page);