On 07.01.25 05:35, Yu Zhao wrote:
Using x86_64 as an example, for a 32KB struct page[] area describing a
2MB hugeTLB, HVO reduces the area to 4KB by the following steps:
1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs;
2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped
by PTE 0, and at the same time change the permission from r/w to
r/o;
3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB
to 4KB.
However, the following race can happen due to improperly memory loads
ordering:
CPU 1 (HVO) CPU 2 (speculative PFN walker)
page_ref_freeze()
synchronize_rcu()
rcu_read_lock()
page_is_fake_head() is false
vmemmap_remap_pte()
XXX: struct page[] becomes r/o
page_ref_unfreeze()
page_ref_count() is not zero
atomic_add_unless(&page->_refcount)
XXX: try to modify r/o struct page[]
Specifically, page_is_fake_head() must be ordered after
page_ref_count() on CPU 2 so that it can only return true for this
case, to avoid the later attempt to modify r/o struct page[].
I *think* this is correct.
This patch adds the missing memory barrier and makes the tests on
page_is_fake_head() and page_ref_count() done in the proper order.
Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
Reported-by: Will Deacon <will@xxxxxxxxxx>
Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
---
include/linux/page-flags.h | 2 +-
include/linux/page_ref.h | 8 ++++++--
2 files changed, 7 insertions(+), 3 deletions(-)
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 691506bdf2c5..6b8ecf86f1b6 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -212,7 +212,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
* cold cacheline in some cases.
*/
if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
- test_bit(PG_head, &page->flags)) {
+ test_bit_acquire(PG_head, &page->flags)) {
This change will affect all page_fixed_fake_head() users, like ordinary
PageTail even on !hugetlb.
I assume you want an explicit memory barrier in the single problematic
caller instead.
--
Cheers,
David / dhildenb
