thp: update futex compound knowledge

Futex code is smarter than most other gup_fast O_DIRECT code and knows
about the compound internals.  However now doing a put_page(head_page)
will not release the pin on the tail page taken by gup-fast, leading to
all sort of refcounting bugchecks.  Getting a stable head_page is a little
tricky.

page_head = page is there because if this is not a tail page it's also the
page_head.  Only in case this is a tail page, compound_head is called,
otherwise it's guaranteed unnecessary.  And if it's a tail page
compound_head has to run atomically inside irq disabled section
__get_user_pages_fast before returning.  Otherwise ->first_page won't be a
stable pointer.

Disableing irq before __get_user_page_fast and releasing irq after running
compound_head is needed because if __get_user_page_fast returns == 1, it
means the huge pmd is established and cannot go away from under us.
pmdp_splitting_flush_notify in __split_huge_page_splitting will have to
wait for local_irq_enable before the IPI delivery can return.  This means
__split_huge_page_refcount can't be running from under us, and in turn
when we run compound_head(page) we're not reading a dangling pointer from
tailpage->first_page.  Then after we get to stable head page, we are
always safe to call compound_lock and after taking the compound lock on
head page we can finally re-check if the page returned by gup-fast is
still a tail page.  in which case we're set and we didn't need to split
the hugepage in order to take a futex on it.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 45 insertions, 10 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 3019b92e6917..52075633373f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -233,7 +233,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 {
         unsigned long address = (unsigned long)uaddr;
         struct mm_struct *mm = current->mm;
-        struct page *page;
+        struct page *page, *page_head;
         int err;
 
         /*
@@ -265,11 +265,46 @@ again:
         if (err < 0)
                 return err;
 
-        page = compound_head(page);
-        lock_page(page);
-        if (!page->mapping) {
-                unlock_page(page);
+#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, 1, &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);
+        }
+#endif
+
+        lock_page(page_head);
+        if (!page_head->mapping) {
+                unlock_page(page_head);
+                put_page(page_head);
                 goto again;
         }
 
@@ -280,20 +315,20 @@ again:
          * it's a read-only handle, it's expected that futexes attach to
          * the object not the particular process.
          */
-        if (PageAnon(page)) {
+        if (PageAnon(page_head)) {
                 key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
                 key->private.mm = mm;
                 key->private.address = address;
         } else {
                 key->both.offset |= FUT_OFF_INODE; /* inode-based key */
-                key->shared.inode = page->mapping->host;
-                key->shared.pgoff = page->index;
+                key->shared.inode = page_head->mapping->host;
+                key->shared.pgoff = page_head->index;
         }
 
         get_futex_key_refs(key);
 
-        unlock_page(page);
-        put_page(page);
+        unlock_page(page_head);
+        put_page(page_head);
         return 0;
 }