mm: drop tail page refcounting

Tail page refcounting is utterly complicated and painful to support.

It uses ->_mapcount on tail pages to store how many times this page is
pinned.  get_page() bumps ->_mapcount on tail page in addition to
->_count on head.  This information is required by split_huge_page() to
be able to distribute pins from head of compound page to tails during
the split.

We will need ->_mapcount to account PTE mappings of subpages of the
compound page.  We eliminate need in current meaning of ->_mapcount in
tail pages by forbidding split entirely if the page is pinned.

The only user of tail page refcounting is THP which is marked BROKEN for
now.

Let's drop all this mess.  It makes get_page() and put_page() much
simpler.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 44 deletions

diff --git a/mm/internal.h b/mm/internal.h
index 38e24b89e4c4..569facd1f6da 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -66,50 +66,6 @@ static inline void set_page_refcounted(struct page *page)
 	set_page_count(page, 1);
 }
 
-static inline void __get_page_tail_foll(struct page *page,
-					bool get_page_head)
-{
-	/*
-	 * If we're getting a tail page, the elevated page->_count is
-	 * required only in the head page and we will elevate the head
-	 * page->_count and tail page->_mapcount.
-	 *
-	 * We elevate page_tail->_mapcount for tail pages to force
-	 * page_tail->_count to be zero at all times to avoid getting
-	 * false positives from get_page_unless_zero() with
-	 * speculative page access (like in
-	 * page_cache_get_speculative()) on tail pages.
-	 */
-	VM_BUG_ON_PAGE(atomic_read(&compound_head(page)->_count) <= 0, page);
-	if (get_page_head)
-		atomic_inc(&compound_head(page)->_count);
-	get_huge_page_tail(page);
-}
-
-/*
- * This is meant to be called as the FOLL_GET operation of
- * follow_page() and it must be called while holding the proper PT
- * lock while the pte (or pmd_trans_huge) is still mapping the page.
- */
-static inline void get_page_foll(struct page *page)
-{
-	if (unlikely(PageTail(page)))
-		/*
-		 * This is safe only because
-		 * __split_huge_page_refcount() can't run under
-		 * get_page_foll() because we hold the proper PT lock.
-		 */
-		__get_page_tail_foll(page, true);
-	else {
-		/*
-		 * Getting a normal page or the head of a compound page
-		 * requires to already have an elevated page->_count.
-		 */
-		VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page);
-		atomic_inc(&page->_count);
-	}
-}
-
 extern unsigned long highest_memmap_pfn;
 
 /*