[PATCH 1/3] mm: hugetlbfs: fix hugetlbfs optimization

Andrea Arcangeli
2013-11-15T17:50:11Z

The patch from commit 7cb2ef56e6a8b7b368b2e883a0a47d02fed66911 can
cause dereference of a dangling pointer if split_huge_page runs during
PageHuge() if there are updates to the tail_page->private field.

Also it is repeating compound_head twice for hugetlbfs and it is
running compound_head+compound_trans_head for THP when a single one is
needed in both cases.

The new code within the PageSlab() check doesn't need to verify that
the THP page size is never bigger than the smallest hugetlbfs page
size, to avoid memory corruption.

A longstanding theoretical race condition was found while fixing the
above (see the change right after the skip_unlock label, that is
relevant for the compound_lock path too).

By re-establishing the _mapcount tail refcounting for all compound
pages, this also fixes the below problem:

echo 0 >/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

BUG: Bad page state in process bash pfn:59a01
page:ffffea000139b038 count:0 mapcount:10 mapping: (null) index:0x0
page flags: 0x1c00000000008000(tail)
Modules linked in:
CPU: 6 PID: 2018 Comm: bash Not tainted 3.12.0+ #25
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
0000000000000009 ffff880079cb5cc8 ffffffff81640e8b 0000000000000006
ffffea000139b038 ffff880079cb5ce8 ffffffff8115bb15 00000000000002c1
ffffea000139b038 ffff880079cb5d48 ffffffff8115bd83 ffff880079cb5de8
Call Trace:
[<ffffffff81640e8b>] dump_stack+0x55/0x76
[<ffffffff8115bb15>] bad_page+0xd5/0x130
[<ffffffff8115bd83>] free_pages_prepare+0x213/0x280
[<ffffffff8115df16>] __free_pages+0x36/0x80
[<ffffffff8119b011>] update_and_free_page+0xc1/0xd0
[<ffffffff8119b512>] free_pool_huge_page+0xc2/0xe0
[<ffffffff8119b8cc>] set_max_huge_pages.part.58+0x14c/0x220
[<ffffffff81308a8c>] ? _kstrtoull+0x2c/0x90
[<ffffffff8119ba70>] nr_hugepages_store_common.isra.60+0xd0/0xf0
[<ffffffff8119bac3>] nr_hugepages_store+0x13/0x20
[<ffffffff812f763f>] kobj_attr_store+0xf/0x20
[<ffffffff812354e9>] sysfs_write_file+0x189/0x1e0
[<ffffffff811baff5>] vfs_write+0xc5/0x1f0
[<ffffffff811bb505>] SyS_write+0x55/0xb0
[<ffffffff81651712>] system_call_fastpath+0x16/0x1b

Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
include/linux/hugetlb.h | 6 ++
mm/hugetlb.c | 17 ++++++
mm/swap.c | 143 ++++++++++++++++++++++++++++--------------------
3 files changed, 106 insertions(+), 60 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index acd2010..d4f3dbf 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -31,6 +31,7 @@ struct hugepage_subpool *hugepage_new_subpool(long nr_blocks);
void hugepage_put_subpool(struct hugepage_subpool *spool);

int PageHuge(struct page *page);
+int PageHeadHuge(struct page *page_head);

void reset_vma_resv_huge_pages(struct vm_area_struct *vma);
int hugetlb_sysctl_handler(struct ctl_table *, int, void __user *, size_t *, loff_t *);
@@ -104,6 +105,11 @@ static inline int PageHuge(struct page *page)
return 0;
}

+static inline int PageHeadHuge(struct page *page_head)
+{
+ return 0;
+}
+
static inline void reset_vma_resv_huge_pages(struct vm_area_struct *vma)
{
}
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7d57af2..14737f8e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -736,6 +736,23 @@ int PageHuge(struct page *page)
}
EXPORT_SYMBOL_GPL(PageHuge);

+/*
+ * PageHeadHuge() only returns true for hugetlbfs head page, but not for
+ * normal or transparent huge pages.
+ */
+int PageHeadHuge(struct page *page_head)
+{
+ compound_page_dtor *dtor;
+
+ if (!PageHead(page_head))
+ return 0;
+
+ dtor = get_compound_page_dtor(page_head);
+
+ return dtor == free_huge_page;
+}
+EXPORT_SYMBOL_GPL(PageHeadHuge);
+
pgoff_t __basepage_index(struct page *page)
{
struct page *page_head = compound_head(page);
diff --git a/mm/swap.c b/mm/swap.c
index 7a9f80d..84b26aa 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -82,19 +82,6 @@ static void __put_compound_page(struct page *page)

static void put_compound_page(struct page *page)
{
- /*
- * hugetlbfs pages cannot be split from under us. If this is a
- * hugetlbfs page, check refcount on head page and release the page if
- * the refcount becomes zero.
- */
- if (PageHuge(page)) {
- page = compound_head(page);
- if (put_page_testzero(page))
- __put_compound_page(page);
-
- return;
- }
-
if (unlikely(PageTail(page))) {
/* __split_huge_page_refcount can run under us */
struct page *page_head = compound_trans_head(page);
@@ -111,14 +98,31 @@ static void put_compound_page(struct page *page)
* still hot on arches that do not support
* this_cpu_cmpxchg_double().
*/
- if (PageSlab(page_head)) {
- if (PageTail(page)) {
+ if (PageSlab(page_head) || PageHeadHuge(page_head)) {
+ if (likely(PageTail(page))) {
+ /*
+ * __split_huge_page_refcount
+ * cannot race here.
+ */
+ VM_BUG_ON(!PageHead(page_head));
+ atomic_dec(&page->_mapcount);
if (put_page_testzero(page_head))
VM_BUG_ON(1);
-
- atomic_dec(&page->_mapcount);
- goto skip_lock_tail;
+ if (put_page_testzero(page_head))
+ __put_compound_page(page_head);
+ return;
} else
+ /*
+ * __split_huge_page_refcount
+ * run before us, "page" was a
+ * THP tail. The split
+ * page_head has been freed
+ * and reallocated as slab or
+ * hugetlbfs page of smaller
+ * order (only possible if
+ * reallocated as slab on
+ * x86).
+ */
goto skip_lock;
}
/*
@@ -132,8 +136,27 @@ static void put_compound_page(struct page *page)
/* __split_huge_page_refcount run before us */
compound_unlock_irqrestore(page_head, flags);
skip_lock:
- if (put_page_testzero(page_head))
- __put_single_page(page_head);
+ if (put_page_testzero(page_head)) {
+ /*
+ * The head page may have been
+ * freed and reallocated as a
+ * compound page of smaller
+ * order and then freed again.
+ * All we know is that it
+ * cannot have become: a THP
+ * page, a compound page of
+ * higher order, a tail page.
+ * That is because we still
+ * hold the refcount of the
+ * split THP tail and
+ * page_head was the THP head
+ * before the split.
+ */
+ if (PageHead(page_head))
+ __put_compound_page(page_head);
+ else
+ __put_single_page(page_head);
+ }
out_put_single:
if (put_page_testzero(page))
__put_single_page(page);
@@ -155,7 +178,6 @@ out_put_single:
VM_BUG_ON(atomic_read(&page->_count) != 0);
compound_unlock_irqrestore(page_head, flags);

-skip_lock_tail:
if (put_page_testzero(page_head)) {
if (PageHead(page_head))
__put_compound_page(page_head);
@@ -198,51 +220,52 @@ bool __get_page_tail(struct page *page)
* proper PT lock that already serializes against
* split_huge_page().
*/
+ unsigned long flags;
bool got = false;
- struct page *page_head;
-
- /*
- * If this is a hugetlbfs page it cannot be split under us. Simply
- * increment refcount for the head page.
- */
- if (PageHuge(page)) {
- page_head = compound_head(page);
- atomic_inc(&page_head->_count);
- got = true;
- } else {
- unsigned long flags;
+ struct page *page_head = compound_trans_head(page);

- page_head = compound_trans_head(page);
- if (likely(page != page_head &&
- get_page_unless_zero(page_head))) {
-
- /* Ref to put_compound_page() comment. */
- if (PageSlab(page_head)) {
- if (likely(PageTail(page))) {
- __get_page_tail_foll(page, false);
- return true;
- } else {
- put_page(page_head);
- return false;
- }
- }
-
- /*
- * page_head wasn't a dangling pointer but it
- * may not be a head page anymore by the time
- * we obtain the lock. That is ok as long as it
- * can't be freed from under us.
- */
- flags = compound_lock_irqsave(page_head);
- /* here __split_huge_page_refcount won't run anymore */
+ if (likely(page != page_head && get_page_unless_zero(page_head))) {
+ /* Ref to put_compound_page() comment. */
+ if (PageSlab(page_head) || PageHeadHuge(page_head)) {
if (likely(PageTail(page))) {
+ /*
+ * This is a hugetlbfs page or a slab
+ * page. __split_huge_page_refcount
+ * cannot race here.
+ */
+ VM_BUG_ON(!PageHead(page_head));
__get_page_tail_foll(page, false);
- got = true;
- }
- compound_unlock_irqrestore(page_head, flags);
- if (unlikely(!got))
+ return true;
+ } else {
+ /*
+ * __split_huge_page_refcount run
+ * before us, "page" was a THP
+ * tail. The split page_head has been
+ * freed and reallocated as slab or
+ * hugetlbfs page of smaller order
+ * (only possible if reallocated as
+ * slab on x86).
+ */
put_page(page_head);
+ return false;
+ }
+ }
+
+ /*
+ * page_head wasn't a dangling pointer but it
+ * may not be a head page anymore by the time
+ * we obtain the lock. That is ok as long as it
+ * can't be freed from under us.
+ */
+ flags = compound_lock_irqsave(page_head);
+ /* here __split_huge_page_refcount won't run anymore */
+ if (likely(PageTail(page))) {
+ __get_page_tail_foll(page, false);
+ got = true;
}
+ compound_unlock_irqrestore(page_head, flags);
+ if (unlikely(!got))
+ put_page(page_head);
}
return got;
}