linux-kernel - Re: [PATCH] mm/rmap: remove unnecessary page_table

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Date: Tue, 23 Apr 2024 07:53:24 +0000
From: "Yajun Deng" <yajun.deng@...ux.dev>
To: "David Hildenbrand" <david@...hat.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: remove unnecessary page_table_lock

April 22, 2024 at 7:24 PM, "David Hildenbrand" <david@...hat.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.

> 
> I think this change is wrong.
> 
> -- Cheers,
> 
> David / dhildenb
>