linux-kernel - Re: [PATCH] mm/rmap: Move anon_vma initialization to anon_vma

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Message-ID: <fde494a5-f5bc-4d5b-ae73-9fe2106e966c@lucifer.local>
Date: Wed, 16 Apr 2025 13:59:21 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Ye Liu <ye.liu@...ux.dev>
Cc: Harry Yoo <harry.yoo@...cle.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ye Liu <liuye@...inos.cn>, David Hildenbrand <david@...hat.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/rmap: Move anon_vma initialization to anon_vma_ctor()

+cc David, Liam

On Wed, Apr 16, 2025 at 09:34:30AM +0800, Ye Liu wrote:
>
> 在 2025/4/15 19:28, Harry Yoo 写道:
> > On Tue, Apr 15, 2025 at 05:25:48PM +0800, Ye Liu wrote:
> >> From: Ye Liu <liuye@...inos.cn>
> >>
> >> Currently, some initialization of anon_vma is performed in
> >> anon_vma_alloc(). Move the initialization to anon_vma_ctor()
> >> so that all object setup is handled in one place.
> >>
> >> Signed-off-by: Ye Liu <liuye@...inos.cn>
> >> ---
> > NACK unless the patch explains how the object's initial state
> > ('constructed state') is preserved between uses.
> >
> > anon_vma_ctor() is a slab constructor. That means it is called only once
> > when a slab (folio) is allocated, and not called again when an anon_vma
> > is allocated from an existing slab (folio). In other words it is not called
> > everytime an object allocated via kmem_cache_alloc() interface.
> Thank you for the feedback. You're absolutely right — I misunderstood
> how the slab constructor (ctor) works. I had assumed it would be called
> every time an object is allocated via kmem_cache_alloc(), but I now
> realize it is only called once when a new slab is initialized, not on
> every object allocation.
> > This patch looks very dangerous to me and makes me question whether you
> > tested it before submission.
> >
> Appreciate you catching this — and yes, I'll test it more thoroughly
> before submitting other patches.
>
> Drop it.
>
> Thanks,
> Ye
>

I agree with Harry's assessment, also this is something that needs to be cc'd to
other developers - I am more generally working on anon_vma at the moment and
this is vma-adjacent even if slab-ish :)

It isn't your fault that you didn't, we need a MAINTAINERS entry for this which
I will submit, but for any future revision, please do cc Harry, David, Liam,
Vlastimil + myself.

I should be sending a MAINTAINERS patch later today which should correct this
going forward!

Cheers, Lorenzo