April 23, 2024 at 4:18 PM, "David Hildenbrand" <david@xxxxxxxxxx> wrote: > > On 23.04.24 09:53, Yajun Deng wrote: > > > > > April 22, 2024 at 7:24 PM, "David Hildenbrand" <david@xxxxxxxxxx> wrote: > > > > > >> > > > > > > > > On 22.04.24 12:52, Yajun Deng wrote: > > > > > > > page_table_lock is a lock that for page table, we won't change page > > > > table in __anon_vma_prepare(). As we can see, it works well in > > > > anon_vma_clone(). They do the same operation. > > > > > > > > We are reusing mm->page_table_lock to serialize, not the *actual* low-level page table locks that really protect PTEs. > > > > > > With that locking gone, there would be nothing protection vma->anon_vma. > > > > > > Note that anon_vma_clone() is likely called with the mmap_lock held in write mode, which is not the case for __anon_vma_prepare() ... > > > > > > > Yes, anon_vma_clone() is called with the mmap_lock held. I added mmap_assert_write_locked(dst->vm_mm) to prove it. > > > > I added mmap_assert_write_locked(vma->vm_mm) in __anon_vma_prepare() at the same time, it shows __anon_vma_prepare() > > > > is also called with the mmap_lock held too. > > > > Make sure you actually have lockdep built in and enabled. > This is my config. CONFIG_LOCKDEP=n CONFIG_DEBUG_VM=y I did another test. I put mmap_assert_write_locked(mm) before 'set_bit(MMF_OOM_SKIP, &mm->flags)' in mmap.c, it's outside the lock. It will crash when on boot. I think mmap_assert_write_locked() works. > __anon_vma_prepare() is for example called from do_anonymous_page() where we might only hold the mmap_lock in read mode (or not at all IIRC with VMA in read mode). > > -- Cheers, > > David / dhildenb >
