Re: [PATCH] mm/rmap: remove unnecessary page_table_lock

["Yajun Deng" <yajun.deng@linux.dev>]

April 23, 2024 at 4:18 PM, "David Hildenbrand" <david@redhat.com> wrote:



> 
> On 23.04.24 09:53, Yajun Deng wrote:
> 
> > 
> > April 22, 2024 at 7:24 PM, "David Hildenbrand" <david@redhat.com> 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
>