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Subject[PATCHv4 23/24] ksm: split huge pages on follow_page()
Date
Let's split THP with FOLL_SPLIT. Attempting to split them laterk would
always fail bacause we take references on tail pages.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
mm/ksm.c | 58 ++++++++--------------------------------------------------
1 file changed, 8 insertions(+), 50 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index a8a88b0f6f62..8d977f074a74 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -441,20 +441,6 @@ static void break_cow(struct rmap_item *rmap_item)
up_read(&mm->mmap_sem);
}

-static struct page *page_trans_compound_anon(struct page *page)
-{
- if (PageTransCompound(page)) {
- struct page *head = compound_head(page);
- /*
- * head may actually be splitted and freed from under
- * us but it's ok here.
- */
- if (PageAnon(head))
- return head;
- }
- return NULL;
-}
-
static struct page *get_mergeable_page(struct rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
@@ -467,10 +453,10 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
if (!vma)
goto out;

- page = follow_page(vma, addr, FOLL_GET);
+ page = follow_page(vma, addr, FOLL_GET | FOLL_SPLIT);
if (IS_ERR_OR_NULL(page))
goto out;
- if (PageAnon(page) || page_trans_compound_anon(page)) {
+ if (PageAnon(page)) {
flush_anon_page(vma, page, addr);
flush_dcache_page(page);
} else {
@@ -976,35 +962,6 @@ out:
return err;
}

-static int page_trans_compound_anon_split(struct page *page)
-{
- int ret = 0;
- struct page *transhuge_head = page_trans_compound_anon(page);
- if (transhuge_head) {
- /* Get the reference on the head to split it. */
- if (get_page_unless_zero(transhuge_head)) {
- /*
- * Recheck we got the reference while the head
- * was still anonymous.
- */
- if (PageAnon(transhuge_head)) {
- lock_page(transhuge_head);
- ret = split_huge_page(transhuge_head);
- unlock_page(transhuge_head);
- } else
- /*
- * Retry later if split_huge_page run
- * from under us.
- */
- ret = 1;
- put_page(transhuge_head);
- } else
- /* Retry later if split_huge_page run from under us. */
- ret = 1;
- }
- return ret;
-}
-
/*
* try_to_merge_one_page - take two pages and merge them into one
* @vma: the vma that holds the pte pointing to page
@@ -1025,9 +982,10 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,

if (!(vma->vm_flags & VM_MERGEABLE))
goto out;
- if (PageTransCompound(page) && page_trans_compound_anon_split(page))
- goto out;
+
+ /* huge pages must be split by this time */
BUG_ON(PageTransCompound(page));
+
if (!PageAnon(page))
goto out;

@@ -1616,14 +1574,14 @@ next_mm:
while (ksm_scan.address < vma->vm_end) {
if (ksm_test_exit(mm))
break;
- *page = follow_page(vma, ksm_scan.address, FOLL_GET);
+ *page = follow_page(vma, ksm_scan.address,
+ FOLL_GET | FOLL_SPLIT);
if (IS_ERR_OR_NULL(*page)) {
ksm_scan.address += PAGE_SIZE;
cond_resched();
continue;
}
- if (PageAnon(*page) ||
- page_trans_compound_anon(*page)) {
+ if (PageAnon(*page)) {
flush_anon_page(vma, *page, ksm_scan.address);
flush_dcache_page(*page);
rmap_item = get_next_rmap_item(slot,
--
2.1.4


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