Re: [RFC PATCH v2 00/13] mm: rework anon_vma and remove anon_vma_chain

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/7/26 11:20, wangtao wrote:
>>> I saw it, and I’ve waited for a while as well.
>>
>> Apparently you saw it but you didn't read my reply about slapping more
>> complexity on something complicated.
> This v2 is a complete rewrite replacing the original anon_vma and anonymous-page
> reverse-mapping traversal.

Okay, it leaves the old code in place for now. Just as a general node without
going into details: that won't fly. We don't need another MGLRU.

> 
> The new anon_vma hierarchy is represented using a simple doubly linked list with depth information,
> replacing the complex topology of red-black trees maintained by each anon_vma in the original design,
> and also removing anon_vma_chain.

Okay, but anon_vma fundamentally remains. Using a lipstick on a pig.

> 
> Excluding patch 11, which optimizes leaf VMA rmap with PVL, and patch 12, which optimizes memory
> usage through shared semaphores, the base version contains roughly 800 lines of code, including the
> compatibility part replacing the original anon_vma implementation.
> 
>>
>>  12 files changed, 1056 insertions(+), 54 deletions(-)
>>
>>>
>>> If there are technical issues, please point them out directly.
>>> Or is this simply how technical discussions are usually conducted in the
>> Linux community or the mm subsystem?
>>
>> I suggest you go back and read my email.
> 
> At first, I thought I was the only one who found anon_vma hard to understand. After reading your earlier email,
> I realized that many people also consider it difficult to follow. 

Right, that's how Lorenzo started his work, trying to replace/remove anon_vma
entirely.

So I tried a different approach and implemented
> anon_vma rmap using a simpler doubly linked list plus depth mechanism.

And this is what I don't understand. You were told that Lorenzo is working on
removing anon_vma, yet you decided to work on + send something that reworks
anon_vma?

Also, I am really curious about your expectation: not having a single patch
contributed to MM, even worse, not a single RB tag or anything, yet you rework
one of the most complicated part of MM repeatedly using other complicated
approaches, and immediately start demand technical discussions? After sending an
absolute horrible v1?

I really try to be a nice person, but really, am I dreaming?


Obviously, Lorenzo is still working om his approach, about which there were
discussions at conferences and LWN articles.

Stating "that you waited for a while" is ludicrous given that your old patch set
was posted around one month ago, and a complete redesign -- moving away from
anon_vma -- is not expected to be a short-term thing.

Meanwhile, Lorenzo is sending real cleanups [1] as preparation for bigger
changes. ... which is what we usually expect on such fundamental reworks.

I'm really left clueless here why we anyone should spend time trying to thing
through yet another data structure and yet another locking scheme, for something
we want to remove entirely?

Again, I really try to be a nice person, but something here just doesn't add up.

[1] https://lore.kernel.org/r/cover.1782735110.git.ljs@kernel.org

-- 
Cheers,

David