[PATCHv8 14/32] thp: file pages support for split_huge_page()

Kirill A. Shutemov
2016-05-12T15:48:10Z

Basic scheme is the same as for anon THP.

Main differences:

- File pages are on radix-tree, so we have head->_count offset by
HPAGE_PMD_NR. The count got distributed to small pages during split.

- mapping->tree_lock prevents non-lockless access to pages under split
over radix-tree;

- Lockless access is prevented by setting the head->_count to 0 during
split;

- After split, some pages can be beyond i_size. We drop them from
radix-tree.

- We don't setup migration entries. Just unmap pages. It helps
handling cases when i_size is in the middle of the page: no need
handle unmap pages beyond i_size manually.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
mm/gup.c | 2 +
mm/huge_memory.c | 160 +++++++++++++++++++++++++++++++++++++++----------------
mm/mempolicy.c | 2 +
3 files changed, 119 insertions(+), 45 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 39f751e1cb33..52f091e28f76 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -287,6 +287,8 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
ret = split_huge_page(page);
unlock_page(page);
put_page(page);
+ if (pmd_none(*pmd))
+ return no_page_table(vma, flags);
}

return ret ? ERR_PTR(ret) :
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index cff5bc64d006..5daad812d71e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -31,6 +31,7 @@
#include <linux/userfaultfd_k.h>
#include <linux/page_idle.h>
#include <linux/swapops.h>
+#include <linux/shmem_fs.h>

#include <asm/tlb.h>
#include <asm/pgalloc.h>
@@ -3145,12 +3146,15 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,

static void freeze_page(struct page *page)
{
- enum ttu_flags ttu_flags = TTU_MIGRATION | TTU_IGNORE_MLOCK |
- TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED;
+ enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
+ TTU_RMAP_LOCKED;
int i, ret;

VM_BUG_ON_PAGE(!PageHead(page), page);

+ if (PageAnon(page))
+ ttu_flags |= TTU_MIGRATION;
+
/* We only need TTU_SPLIT_HUGE_PMD once */
ret = try_to_unmap(page, ttu_flags | TTU_SPLIT_HUGE_PMD);
for (i = 1; !ret && i < HPAGE_PMD_NR; i++) {
@@ -3160,7 +3164,7 @@ static void freeze_page(struct page *page)

ret = try_to_unmap(page + i, ttu_flags);
}
- VM_BUG_ON(ret);
+ VM_BUG_ON_PAGE(ret, page + i - 1);
}

static void unfreeze_page(struct page *page)
@@ -3182,15 +3186,20 @@ static void __split_huge_page_tail(struct page *head, int tail,
/*
* tail_page->_count is zero and not changing from under us. But
* get_page_unless_zero() may be running from under us on the
- * tail_page. If we used atomic_set() below instead of atomic_inc(), we
- * would then run atomic_set() concurrently with
+ * tail_page. If we used atomic_set() below instead of atomic_inc() or
+ * atomic_add(), we would then run atomic_set() concurrently with
* get_page_unless_zero(), and atomic_set() is implemented in C not
* using locked ops. spin_unlock on x86 sometime uses locked ops
* because of PPro errata 66, 92, so unless somebody can guarantee
* atomic_set() here would be safe on all archs (and not only on x86),
- * it's safer to use atomic_inc().
+ * it's safer to use atomic_inc()/atomic_add().
*/
- page_ref_inc(page_tail);
+ if (PageAnon(head)) {
+ page_ref_inc(page_tail);
+ } else {
+ /* Additional pin to radix tree */
+ page_ref_add(page_tail, 2);
+ }

page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
page_tail->flags |= (head->flags &
@@ -3226,25 +3235,44 @@ static void __split_huge_page_tail(struct page *head, int tail,
lru_add_page_tail(head, page_tail, lruvec, list);
}

-static void __split_huge_page(struct page *page, struct list_head *list)
+static void __split_huge_page(struct page *page, struct list_head *list,
+ unsigned long flags)
{
struct page *head = compound_head(page);
struct zone *zone = page_zone(head);
struct lruvec *lruvec;
+ pgoff_t end = -1;
int i;

- /* prevent PageLRU to go away from under us, and freeze lru stats */
- spin_lock_irq(&zone->lru_lock);
lruvec = mem_cgroup_page_lruvec(head, zone);

/* complete memcg works before add pages to LRU */
mem_cgroup_split_huge_fixup(head);

- for (i = HPAGE_PMD_NR - 1; i >= 1; i--)
+ if (!PageAnon(page))
+ end = DIV_ROUND_UP(i_size_read(head->mapping->host), PAGE_SIZE);
+
+ for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
__split_huge_page_tail(head, i, lruvec, list);
+ /* Some pages can be beyond i_size: drop them from page cache */
+ if (head[i].index >= end) {
+ __ClearPageDirty(head + i);
+ __delete_from_page_cache(head + i, NULL);
+ put_page(head + i);
+ }
+ }

ClearPageCompound(head);
- spin_unlock_irq(&zone->lru_lock);
+ /* See comment in __split_huge_page_tail() */
+ if (PageAnon(head)) {
+ page_ref_inc(head);
+ } else {
+ /* Additional pin to radix tree */
+ page_ref_add(head, 2);
+ spin_unlock(&head->mapping->tree_lock);
+ }
+
+ spin_unlock_irqrestore(&page_zone(head)->lru_lock, flags);

unfreeze_page(head);

@@ -3311,36 +3339,54 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
{
struct page *head = compound_head(page);
struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
- struct anon_vma *anon_vma;
- int count, mapcount, ret;
+ struct anon_vma *anon_vma = NULL;
+ struct address_space *mapping = NULL;
+ int count, mapcount, extra_pins, ret;
bool mlocked;
unsigned long flags;

VM_BUG_ON_PAGE(is_huge_zero_page(page), page);
- VM_BUG_ON_PAGE(!PageAnon(page), page);
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
VM_BUG_ON_PAGE(!PageCompound(page), page);

- /*
- * The caller does not necessarily hold an mmap_sem that would prevent
- * the anon_vma disappearing so we first we take a reference to it
- * and then lock the anon_vma for write. This is similar to
- * page_lock_anon_vma_read except the write lock is taken to serialise
- * against parallel split or collapse operations.
- */
- anon_vma = page_get_anon_vma(head);
- if (!anon_vma) {
- ret = -EBUSY;
- goto out;
+ if (PageAnon(head)) {
+ /*
+ * The caller does not necessarily hold an mmap_sem that would
+ * prevent the anon_vma disappearing so we first we take a
+ * reference to it and then lock the anon_vma for write. This
+ * is similar to page_lock_anon_vma_read except the write lock
+ * is taken to serialise against parallel split or collapse
+ * operations.
+ */
+ anon_vma = page_get_anon_vma(head);
+ if (!anon_vma) {
+ ret = -EBUSY;
+ goto out;
+ }
+ extra_pins = 0;
+ mapping = NULL;
+ anon_vma_lock_write(anon_vma);
+ } else {
+ mapping = head->mapping;
+
+ /* Truncated ? */
+ if (!mapping) {
+ ret = -EBUSY;
+ goto out;
+ }
+
+ /* Addidional pins from radix tree */
+ extra_pins = HPAGE_PMD_NR;
+ anon_vma = NULL;
+ i_mmap_lock_read(mapping);
}
- anon_vma_lock_write(anon_vma);

/*
* Racy check if we can split the page, before freeze_page() will
* split PMDs
*/
- if (total_mapcount(head) != page_count(head) - 1) {
+ if (total_mapcount(head) != page_count(head) - extra_pins - 1) {
ret = -EBUSY;
goto out_unlock;
}
@@ -3353,35 +3399,60 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
if (mlocked)
lru_add_drain();

+ /* prevent PageLRU to go away from under us, and freeze lru stats */
+ spin_lock_irqsave(&page_zone(head)->lru_lock, flags);
+
+ if (mapping) {
+ void **pslot;
+
+ spin_lock(&mapping->tree_lock);
+ pslot = radix_tree_lookup_slot(&mapping->page_tree,
+ page_index(head));
+ /*
+ * Check if the head page is present in radix tree.
+ * We assume all tail are present too, if head is there.
+ */
+ if (radix_tree_deref_slot_protected(pslot,
+ &mapping->tree_lock) != head)
+ goto fail;
+ }
+
/* Prevent deferred_split_scan() touching ->_count */
- spin_lock_irqsave(&pgdata->split_queue_lock, flags);
+ spin_lock(&pgdata->split_queue_lock);
count = page_count(head);
mapcount = total_mapcount(head);
- if (!mapcount && count == 1) {
+ if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
if (!list_empty(page_deferred_list(head))) {
pgdata->split_queue_len--;
list_del(page_deferred_list(head));
}
- spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
- __split_huge_page(page, list);
+ spin_unlock(&pgdata->split_queue_lock);
+ __split_huge_page(page, list, flags);
ret = 0;
- } else if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
- spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
- pr_alert("total_mapcount: %u, page_count(): %u\n",
- mapcount, count);
- if (PageTail(page))
- dump_page(head, NULL);
- dump_page(page, "total_mapcount(head) > 0");
- BUG();
} else {
- spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
+ if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
+ pr_alert("total_mapcount: %u, page_count(): %u\n",
+ mapcount, count);
+ if (PageTail(page))
+ dump_page(head, NULL);
+ dump_page(page, "total_mapcount(head) > 0");
+ BUG();
+ }
+ spin_unlock(&pgdata->split_queue_lock);
+fail: if (mapping)
+ spin_unlock(&mapping->tree_lock);
+ spin_unlock_irqrestore(&page_zone(head)->lru_lock, flags);
unfreeze_page(head);
ret = -EBUSY;
}

out_unlock:
- anon_vma_unlock_write(anon_vma);
- put_anon_vma(anon_vma);
+ if (anon_vma) {
+ anon_vma_unlock_write(anon_vma);
+ put_anon_vma(anon_vma);
+ }
+ if (mapping)
+ i_mmap_unlock_read(mapping);
out:
count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
return ret;
@@ -3504,8 +3575,7 @@ static int split_huge_pages_set(void *data, u64 val)
if (zone != page_zone(page))
goto next;

- if (!PageHead(page) || !PageAnon(page) ||
- PageHuge(page))
+ if (!PageHead(page) || PageHuge(page) || !PageLRU(page))
goto next;

total++;
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 36cc01bc950a..148143974c5b 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -515,6 +515,8 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
}
}

+ if (pmd_trans_unstable(pmd))
+ return 0;
retry:
pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
for (; addr != end; pte++, addr += PAGE_SIZE) {
--
2.8.1