[PATCH v2] mm, futex: Fix shared futex pgoff on shmem huge page

[Hugh Dickins]

If more than one futex is placed on a shmem huge page, it can happen that
waking the second wakes the first instead, and leaves the second waiting:
the key's shared.pgoff is wrong.

When 3.11 commit 13d60f4b6ab5 ("futex: Take hugepages into account when
generating futex_key"), the only shared huge pages came from hugetlbfs,
and the code added to deal with its exceptional page->index was put into
hugetlb source.  Then that was missed when 4.8 added shmem huge pages.

page_to_pgoff() is what others use for this nowadays: except that, as
currently written, it gives the right answer on hugetlbfs head, but
nonsense on hugetlbfs tails.  Fix that by calling hugetlbfs-specific
hugetlb_basepage_index() on PageHuge tails as well as on head.

Yes, it's unconventional to declare hugetlb_basepage_index() there in
pagemap.h, rather than in hugetlb.h; but I do not expect anything but
page_to_pgoff() ever to need it.

Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
Reported-by: Neel Natu <neelnatu@xxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
v2: restored TODO comments, per Matthew; make "hugetlb pages" explicit.

 include/linux/hugetlb.h |   16 ----------------
 include/linux/pagemap.h |   13 +++++++------
 kernel/futex.c          |    3 +--
 mm/hugetlb.c            |    5 +----
 4 files changed, 9 insertions(+), 28 deletions(-)

--- 5.13-rc5/include/linux/hugetlb.h	2021-05-09 17:25:09.278703159 -0700
+++ linux/include/linux/hugetlb.h	2021-06-11 17:30:28.726720252 -0700
@@ -733,17 +733,6 @@ static inline int hstate_index(struct hs
 	return h - hstates;
 }
 
-pgoff_t __basepage_index(struct page *page);
-
-/* Return page->index in PAGE_SIZE units */
-static inline pgoff_t basepage_index(struct page *page)
-{
-	if (!PageCompound(page))
-		return page->index;
-
-	return __basepage_index(page);
-}
-
 extern int dissolve_free_huge_page(struct page *page);
 extern int dissolve_free_huge_pages(unsigned long start_pfn,
 				    unsigned long end_pfn);
@@ -980,11 +969,6 @@ static inline int hstate_index(struct hs
 	return 0;
 }
 
-static inline pgoff_t basepage_index(struct page *page)
-{
-	return page->index;
-}
-
 static inline int dissolve_free_huge_page(struct page *page)
 {
 	return 0;
--- 5.13-rc5/include/linux/pagemap.h	2021-05-16 22:49:30.036176843 -0700
+++ linux/include/linux/pagemap.h	2021-06-12 19:29:23.364387191 -0700
@@ -516,7 +516,7 @@ static inline struct page *read_mapping_
 }
 
 /*
- * Get index of the page with in radix-tree
+ * Get index of the page within radix-tree (but not for hugetlb pages).
  * (TODO: remove once hugetlb pages will have ->index in PAGE_SIZE)
  */
 static inline pgoff_t page_to_index(struct page *page)
@@ -536,14 +536,15 @@ static inline pgoff_t page_to_index(stru
 }
 
 /*
- * Get the offset in PAGE_SIZE.
- * (TODO: hugepage should have ->index in PAGE_SIZE)
+ * Get the offset in PAGE_SIZE (even for hugetlb pages).
+ * (TODO: hugetlb pages should have ->index in PAGE_SIZE)
  */
 static inline pgoff_t page_to_pgoff(struct page *page)
 {
-	if (unlikely(PageHeadHuge(page)))
-		return page->index << compound_order(page);
-
+	if (unlikely(PageHuge(page))) {
+		extern pgoff_t hugetlb_basepage_index(struct page *page);
+		return hugetlb_basepage_index(page);
+	}
 	return page_to_index(page);
 }
 
--- 5.13-rc5/kernel/futex.c	2021-05-09 17:25:09.670705811 -0700
+++ linux/kernel/futex.c	2021-06-11 17:30:28.726720252 -0700
@@ -35,7 +35,6 @@
 #include <linux/jhash.h>
 #include <linux/pagemap.h>
 #include <linux/syscalls.h>
-#include <linux/hugetlb.h>
 #include <linux/freezer.h>
 #include <linux/memblock.h>
 #include <linux/fault-inject.h>
@@ -650,7 +649,7 @@ again:
 
 		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
 		key->shared.i_seq = get_inode_sequence_number(inode);
-		key->shared.pgoff = basepage_index(tail);
+		key->shared.pgoff = page_to_pgoff(tail);
 		rcu_read_unlock();
 	}
 
--- 5.13-rc5/mm/hugetlb.c	2021-06-06 16:57:26.263006733 -0700
+++ linux/mm/hugetlb.c	2021-06-11 17:30:28.730720276 -0700
@@ -1588,15 +1588,12 @@ struct address_space *hugetlb_page_mappi
 	return NULL;
 }
 
-pgoff_t __basepage_index(struct page *page)
+pgoff_t hugetlb_basepage_index(struct page *page)
 {
 	struct page *page_head = compound_head(page);
 	pgoff_t index = page_index(page_head);
 	unsigned long compound_idx;
 
-	if (!PageHuge(page_head))
-		return page_index(page);
-
 	if (compound_order(page_head) >= MAX_ORDER)
 		compound_idx = page_to_pfn(page) - page_to_pfn(page_head);
 	else