Internal syscaller on linux-next reported a lock inversion cause by
vm_lock being taken after i_mmap_rwsem:
======================================================
WARNING: possible circular locking dependency detected
6.2.0-next-20230301-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor115/5084 is trying to acquire lock:
ffff888078307a90 (&vma->vm_lock->lock){++++}-{3:3}, at: vma_start_write include/linux/mm.h:678 [inline]
ffff888078307a90 (&vma->vm_lock->lock){++++}-{3:3}, at: retract_page_tables mm/khugepaged.c:1826 [inline]
ffff888078307a90 (&vma->vm_lock->lock){++++}-{3:3}, at: collapse_file+0x4fa5/0x5980 mm/khugepaged.c:2204
but task is already holding lock:
ffff88801f93efa8 (&mapping->i_mmap_rwsem){++++}-{3:3}, at: i_mmap_lock_write include/linux/fs.h:468 [inline]
ffff88801f93efa8 (&mapping->i_mmap_rwsem){++++}-{3:3}, at: retract_page_tables mm/khugepaged.c:1745 [inline]
ffff88801f93efa8 (&mapping->i_mmap_rwsem){++++}-{3:3}, at: collapse_file+0x3da6/0x5980 mm/khugepaged.c:2204
retract_page_tables takes i_mmap_rwsem before exclusive mmap_lock, which
is inverse to normal order. Deadlock is avoided by try-locking mmap_lock
and skipping on failure to obtain it. Locking the VMA should use the same
locking pattern to avoid this lock inversion.
Fixes: 44a83f2083bd ("mm/khugepaged: write-lock VMA while collapsing a huge page")
Reported-by: syzbot+8955a9646d1a48b8be92@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
include/linux/mm.h | 39 ++++++++++++++++++++++++++++-----------
mm/khugepaged.c | 5 ++++-
2 files changed, 32 insertions(+), 12 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1b9be34a24fb..5f16263d176d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -676,18 +676,23 @@ static inline void vma_end_read(struct vm_area_struct *vma)
rcu_read_unlock();
}
-static inline void vma_start_write(struct vm_area_struct *vma)
+static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
{
- int mm_lock_seq;
-
mmap_assert_write_locked(vma->vm_mm);
/*
* current task is holding mmap_write_lock, both vma->vm_lock_seq and
* mm->mm_lock_seq can't be concurrently modified.
*/
- mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq);
- if (vma->vm_lock_seq == mm_lock_seq)
+ *mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq);
+ return (vma->vm_lock_seq == *mm_lock_seq);
+}
+
+static inline void vma_start_write(struct vm_area_struct *vma)
+{
+ int mm_lock_seq;
+
+ if (__is_vma_write_locked(vma, &mm_lock_seq))
return;
down_write(&vma->vm_lock->lock);
@@ -695,14 +700,26 @@ static inline void vma_start_write(struct vm_area_struct *vma)
up_write(&vma->vm_lock->lock);
}
+static inline bool vma_try_start_write(struct vm_area_struct *vma)
+{
+ int mm_lock_seq;
+
+ if (__is_vma_write_locked(vma, &mm_lock_seq))
+ return true;
+
+ if (!down_write_trylock(&vma->vm_lock->lock))
+ return false;
+
+ vma->vm_lock_seq = mm_lock_seq;
+ up_write(&vma->vm_lock->lock);
+ return true;
+}
+
static inline void vma_assert_write_locked(struct vm_area_struct *vma)
{
- mmap_assert_write_locked(vma->vm_mm);
- /*
- * current task is holding mmap_write_lock, both vma->vm_lock_seq and
- * mm->mm_lock_seq can't be concurrently modified.
- */
- VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
+ int mm_lock_seq;
+
+ VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
}
static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c64e01f03f27..408fed42c9f5 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1795,6 +1795,10 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
result = SCAN_PTE_MAPPED_HUGEPAGE;
if ((cc->is_khugepaged || is_target) &&
mmap_write_trylock(mm)) {
+ /* trylock for the same lock inversion as above */
+ if (!vma_try_start_write(vma))
+ goto unlock_next;
+
/*
* Re-check whether we have an ->anon_vma, because
* collapse_and_free_pmd() requires that either no
@@ -1823,7 +1827,6 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
result = SCAN_PTE_UFFD_WP;
goto unlock_next;
}
- vma_start_write(vma);
collapse_and_free_pmd(mm, vma, addr, pmd);
if (!cc->is_khugepaged && is_target)
result = set_huge_pmd(vma, addr, pmd, hpage);
--
2.40.0.rc0.216.gc4246ad0f0-goog
