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Subject[PATCH 07 of 66] update futex compound knowledge
Date
From
From: Andrea Arcangeli <aarcange@redhat.com>

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>
---
diff --git a/kernel/futex.c b/kernel/futex.c
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -219,7 +219,7 @@ get_futex_key(u32 __user *uaddr, int fsh
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
- struct page *page;
+ struct page *page, *page_head;
int err;

/*
@@ -251,11 +251,46 @@ 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;
}

@@ -266,20 +301,20 @@ 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;
}


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