v3-v4:
- Added comments.
- Removed VM_BUG_ON on PageHead from put_page.
v2-v3:
- Check if page is still compound page after inc refcnt.
v1-v2:
- Avoid taking compound lock for slab pages.
--8<--------------------------cut here-------------------------->8--
Transparent huge pages can change page->flags (PG_compound_lock)
without taking Slab lock. Since THP can not break slab pages we can
safely access compound page without taking compound lock.
Specifically this patch fixes race between compound_unlock and slab
functions which does page-flags update. This can occur when
get_page/put_page is called on page from slab object.
Reported-by: Amey Bhide <[email protected]>
Signed-off-by: Pravin B Shelar <[email protected]>
Reviewed-by: Christoph Lameter <[email protected]>
---
include/linux/mm.h | 2 ++
mm/swap.c | 34 +++++++++++++++++++++++++++++++++-
2 files changed, 35 insertions(+), 1 deletion(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8437e93..ddd58ce 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -321,6 +321,7 @@ static inline int is_vmalloc_or_module_addr(const void *x)
static inline void compound_lock(struct page *page)
{
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+ VM_BUG_ON(PageSlab(page));
bit_spin_lock(PG_compound_lock, &page->flags);
#endif
}
@@ -328,6 +329,7 @@ static inline void compound_lock(struct page *page)
static inline void compound_unlock(struct page *page)
{
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+ VM_BUG_ON(PageSlab(page));
bit_spin_unlock(PG_compound_lock, &page->flags);
#endif
}
diff --git a/mm/swap.c b/mm/swap.c
index 8ff73d8..93c709b 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -82,6 +82,24 @@ static void put_compound_page(struct page *page)
if (likely(page != page_head &&
get_page_unless_zero(page_head))) {
unsigned long flags;
+
+ /* THP can not break up slab pages, avoid taking
+ * compound_lock(). Slab prefer non atomic bit ops
+ * on page->flags for better performance. In particular
+ * slab_unlock() in slub used to be a hot path
+ * item. It is still hot on arches that do not support
+ * this_cpu_cmpxchg_double. */
+
+ if (PageSlab(page_head)) {
+ if (PageTail(page)) {
+ if (put_page_testzero(page_head))
+ VM_BUG_ON(1);
+
+ atomic_dec(&page->_mapcount);
+ goto skip_lock_tail;
+ } else
+ goto skip_lock;
+ }
/*
* page_head wasn't a dangling pointer but it
* may not be a head page anymore by the time
@@ -92,7 +110,7 @@ static void put_compound_page(struct page *page)
if (unlikely(!PageTail(page))) {
/* __split_huge_page_refcount run before us */
compound_unlock_irqrestore(page_head, flags);
- VM_BUG_ON(PageHead(page_head));
+ skip_lock:
if (put_page_testzero(page_head))
__put_single_page(page_head);
out_put_single:
@@ -115,6 +133,8 @@ static void put_compound_page(struct page *page)
VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
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);
@@ -162,6 +182,18 @@ bool __get_page_tail(struct page *page)
struct 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
--
1.7.10