While investigating HVO for THPs [1], it turns out that speculative
PFN walkers like compaction can race with vmemmap modifications, e.g.,
CPU 1 (vmemmap modifier) CPU 2 (speculative PFN walker)
------------------------------- ------------------------------
Allocates an LRU folio page1
Sees page1
Frees page1
Allocates a hugeTLB folio page2
(page1 being a tail of page2)
Updates vmemmap mapping page1
get_page_unless_zero(page1)
Even though page1->_refcount is zero after HVO, get_page_unless_zero()
can still try to modify this read-only field, resulting in a crash.
An independent report [2] confirmed this race.
There are two discussed approaches to fix this race:
1. Make RO vmemmap RW so that get_page_unless_zero() can fail without
triggering a PF.
2. Use RCU to make sure get_page_unless_zero() either sees zero
page->_refcount through the old vmemmap or non-zero page->_refcount
through the new one.
The second approach is preferred here because:
1. It can prevent illegal modifications to struct page[] that has been
HVO'ed;
2. It can be generalized, in a way similar to ZERO_PAGE(), to fix
similar races in other places, e.g., arch_remove_memory() on x86
[3], which frees vmemmap mapping offlined struct page[].
While adding synchronize_rcu(), the goal is to be surgical, rather
than optimized. Specifically, calls to synchronize_rcu() on the error
handling paths can be coalesced, but it is not done for the sake of
Simplicity: noticeably, this fix removes ~50% more lines than it adds.
According to the hugetlb_optimize_vmemmap section in
Documentation/admin-guide/sysctl/vm.rst, enabling HVO makes allocating
or freeing hugeTLB pages "~2x slower than before". Having
synchronize_rcu() on top makes those operations even worse, and this
also affects the user interface /proc/sys/vm/nr_overcommit_hugepages.
[1] https://lore.kernel.org/20240229183436.4110845-4-yuzhao@xxxxxxxxxx/
[2] https://lore.kernel.org/917FFC7F-0615-44DD-90EE-9F85F8EA9974@xxxxxxxxx/
[3] https://lore.kernel.org/be130a96-a27e-4240-ad78-776802f57cad@xxxxxxxxxx/
Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Acked-by: Muchun Song <muchun.song@xxxxxxxxx>
---
include/linux/page_ref.h | 8 +++++-
mm/hugetlb.c | 53 ++++++----------------------------------
mm/hugetlb_vmemmap.c | 16 ++++++++++++
3 files changed, 30 insertions(+), 47 deletions(-)
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 490d0ad6e56d..8c236c651d1d 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -230,7 +230,13 @@ static inline int folio_ref_dec_return(struct folio *folio)
static inline bool page_ref_add_unless(struct page *page, int nr, int u)
{
- bool ret = atomic_add_unless(&page->_refcount, nr, u);
+ bool ret = false;
+
+ rcu_read_lock();
+ /* avoid writing to the vmemmap area being remapped */
+ if (!page_is_fake_head(page) && page_ref_count(page) != u)
+ ret = atomic_add_unless(&page->_refcount, nr, u);
+ rcu_read_unlock();
if (page_ref_tracepoint_active(page_ref_mod_unless))
__page_ref_mod_unless(page, nr, ret);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9691624fcb79..0a69e194b517 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1629,13 +1629,10 @@ static inline void destroy_compound_gigantic_folio(struct folio *folio,
* folio appears as just a compound page. Otherwise, wait until after
* allocating vmemmap to clear the flag.
*
- * A reference is held on the folio, except in the case of demote.
- *
* Must be called with hugetlb lock held.
*/
-static void __remove_hugetlb_folio(struct hstate *h, struct folio *folio,
- bool adjust_surplus,
- bool demote)
+static void remove_hugetlb_folio(struct hstate *h, struct folio *folio,
+ bool adjust_surplus)
{
int nid = folio_nid(folio);
@@ -1649,6 +1646,7 @@ static void __remove_hugetlb_folio(struct hstate *h, struct folio *folio,
list_del(&folio->lru);
if (folio_test_hugetlb_freed(folio)) {
+ folio_clear_hugetlb_freed(folio);
h->free_huge_pages--;
h->free_huge_pages_node[nid]--;
}
@@ -1665,33 +1663,13 @@ static void __remove_hugetlb_folio(struct hstate *h, struct folio *folio,
if (!folio_test_hugetlb_vmemmap_optimized(folio))
__folio_clear_hugetlb(folio);
- /*
- * In the case of demote we do not ref count the page as it will soon
- * be turned into a page of smaller size.
- */
- if (!demote)
- folio_ref_unfreeze(folio, 1);
-
h->nr_huge_pages--;
h->nr_huge_pages_node[nid]--;
}
-static void remove_hugetlb_folio(struct hstate *h, struct folio *folio,
- bool adjust_surplus)
-{
- __remove_hugetlb_folio(h, folio, adjust_surplus, false);
-}
-
-static void remove_hugetlb_folio_for_demote(struct hstate *h, struct folio *folio,
- bool adjust_surplus)
-{
- __remove_hugetlb_folio(h, folio, adjust_surplus, true);
-}
-
static void add_hugetlb_folio(struct hstate *h, struct folio *folio,
bool adjust_surplus)
{
- int zeroed;
int nid = folio_nid(folio);
VM_BUG_ON_FOLIO(!folio_test_hugetlb_vmemmap_optimized(folio), folio);
@@ -1715,21 +1693,6 @@ static void add_hugetlb_folio(struct hstate *h, struct folio *folio,
*/
folio_set_hugetlb_vmemmap_optimized(folio);
- /*
- * This folio is about to be managed by the hugetlb allocator and
- * should have no users. Drop our reference, and check for others
- * just in case.
- */
- zeroed = folio_put_testzero(folio);
- if (unlikely(!zeroed))
- /*
- * It is VERY unlikely soneone else has taken a ref
- * on the folio. In this case, we simply return as
- * free_huge_folio() will be called when this other ref
- * is dropped.
- */
- return;
-
arch_clear_hugetlb_flags(folio);
enqueue_hugetlb_folio(h, folio);
}
@@ -1783,6 +1746,8 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
spin_unlock_irq(&hugetlb_lock);
}
+ folio_ref_unfreeze(folio, 1);
+
/*
* Non-gigantic pages demoted from CMA allocated gigantic pages
* need to be given back to CMA in free_gigantic_folio.
@@ -3106,11 +3071,8 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
free_new:
spin_unlock_irq(&hugetlb_lock);
- if (new_folio) {
- /* Folio has a zero ref count, but needs a ref to be freed */
- folio_ref_unfreeze(new_folio, 1);
+ if (new_folio)
update_and_free_hugetlb_folio(h, new_folio, false);
- }
return ret;
}
@@ -3965,7 +3927,7 @@ static int demote_free_hugetlb_folio(struct hstate *h, struct folio *folio)
target_hstate = size_to_hstate(PAGE_SIZE << h->demote_order);
- remove_hugetlb_folio_for_demote(h, folio, false);
+ remove_hugetlb_folio(h, folio, false);
spin_unlock_irq(&hugetlb_lock);
/*
@@ -3979,7 +3941,6 @@ static int demote_free_hugetlb_folio(struct hstate *h, struct folio *folio)
if (rc) {
/* Allocation of vmemmmap failed, we can not demote folio */
spin_lock_irq(&hugetlb_lock);
- folio_ref_unfreeze(folio, 1);
add_hugetlb_folio(h, folio, false);
return rc;
}
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index fa00d61b6c5a..829112b0a914 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -455,6 +455,8 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
unsigned long vmemmap_reuse;
VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
+ VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);
+
if (!folio_test_hugetlb_vmemmap_optimized(folio))
return 0;
@@ -490,6 +492,9 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
*/
int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
{
+ /* avoid writes from page_ref_add_unless() while unfolding vmemmap */
+ synchronize_rcu();
+
return __hugetlb_vmemmap_restore_folio(h, folio, 0);
}
@@ -514,6 +519,9 @@ long hugetlb_vmemmap_restore_folios(const struct hstate *h,
long restored = 0;
long ret = 0;
+ /* avoid writes from page_ref_add_unless() while unfolding vmemmap */
+ synchronize_rcu();
+
list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
if (folio_test_hugetlb_vmemmap_optimized(folio)) {
ret = __hugetlb_vmemmap_restore_folio(h, folio,
@@ -559,6 +567,8 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
unsigned long vmemmap_reuse;
VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
+ VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);
+
if (!vmemmap_should_optimize_folio(h, folio))
return ret;
@@ -610,6 +620,9 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
{
LIST_HEAD(vmemmap_pages);
+ /* avoid writes from page_ref_add_unless() while folding vmemmap */
+ synchronize_rcu();
+
__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0);
free_vmemmap_page_list(&vmemmap_pages);
}
@@ -653,6 +666,9 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_l
flush_tlb_all();
+ /* avoid writes from page_ref_add_unless() while folding vmemmap */
+ synchronize_rcu();
+
list_for_each_entry(folio, folio_list, lru) {
int ret;
--
2.45.2.803.g4e1b14247a-goog
