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

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Date: Tue, 23 Apr 2024 10:18:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Yajun Deng <yajun.deng@...ux.dev>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: remove unnecessary page_table_lock

On 23.04.24 09:53, Yajun Deng wrote:
> 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.

Make sure you actually have lockdep built in and enabled.

__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