Re: [PATCHv5 14/28] futex, thp: remove special case for THP in get_futex_key

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On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
With new THP refcounting, we don't need tricks to stabilize huge page.
If we've got reference to tail page, it can't split under us.

This patch effectively reverts a5b338f2b0b1.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
  kernel/futex.c | 61 ++++++++++++----------------------------------------------
  1 file changed, 12 insertions(+), 49 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index f4d8a85641ed..cf0192e60ef9 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -399,7 +399,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
  {
  	unsigned long address = (unsigned long)uaddr;
  	struct mm_struct *mm = current->mm;
-	struct page *page, *page_head;
+	struct page *page;
  	int err, ro = 0;

  	/*
@@ -442,46 +442,9 @@ again:
  	else
  		err = 0;

-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	page_head = page;
-	if (unlikely(PageTail(page))) {
-		put_page(page);
-		/* serialize against __split_huge_page_splitting() */
-		local_irq_disable();
-		if (likely(__get_user_pages_fast(address, 1, !ro, &page) == 1)) {
-			page_head = compound_head(page);
-			/*
-			 * page_head is valid pointer but we must pin
-			 * it before taking the PG_lock and/or
-			 * PG_compound_lock. The moment we re-enable
-			 * irqs __split_huge_page_splitting() can
-			 * return and the head page can be freed from
-			 * under us. We can't take the PG_lock and/or
-			 * PG_compound_lock on a page that could be
-			 * freed from under us.
-			 */
-			if (page != page_head) {
-				get_page(page_head);
-				put_page(page);
-			}
-			local_irq_enable();
-		} else {
-			local_irq_enable();
-			goto again;
-		}
-	}
-#else
-	page_head = compound_head(page);
-	if (page != page_head) {
-		get_page(page_head);
-		put_page(page);
-	}

Hmm, any idea why this was there? Without THP, it was already sure that get/put_page() on tail page operates on the head page's _count, no?

-#endif
-
-	lock_page(page_head);
-
+	lock_page(page);
  	/*
-	 * If page_head->mapping is NULL, then it cannot be a PageAnon
+	 * If page->mapping is NULL, then it cannot be a PageAnon
  	 * page; but it might be the ZERO_PAGE or in the gate area or
  	 * in a special mapping (all cases which we are happy to fail);
  	 * or it may have been a good file page when get_user_pages_fast
@@ -493,12 +456,12 @@ again:
  	 *
  	 * The case we do have to guard against is when memory pressure made
  	 * shmem_writepage move it from filecache to swapcache beneath us:
-	 * an unlikely race, but we do need to retry for page_head->mapping.
+	 * an unlikely race, but we do need to retry for page->mapping.
  	 */
-	if (!page_head->mapping) {
-		int shmem_swizzled = PageSwapCache(page_head);
-		unlock_page(page_head);
-		put_page(page_head);
+	if (!page->mapping) {
+		int shmem_swizzled = PageSwapCache(page);
+		unlock_page(page);
+		put_page(page);
  		if (shmem_swizzled)
  			goto again;
  		return -EFAULT;
@@ -511,7 +474,7 @@ again:
  	 * it's a read-only handle, it's expected that futexes attach to
  	 * the object not the particular process.
  	 */
-	if (PageAnon(page_head)) {
+	if (PageAnon(page)) {
  		/*
  		 * A RO anonymous page will never change and thus doesn't make
  		 * sense for futex operations.
@@ -526,15 +489,15 @@ again:
  		key->private.address = address;
  	} else {
  		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
-		key->shared.inode = page_head->mapping->host;
+		key->shared.inode = page->mapping->host;
  		key->shared.pgoff = basepage_index(page);
  	}

  	get_futex_key_refs(key); /* implies MB (B) */

  out:
-	unlock_page(page_head);
-	put_page(page_head);
+	unlock_page(page);
+	put_page(page);
  	return err;
  }



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