linux-kernel - Re: [PATCH v5 3/6] mm: mark vma as detached until it's added into vma tree

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Message-ID: <CAJuCfpHVj+iVBd1O3wPFFSxanxk_b+Ym=qf4QE0U2HaJGbNdgg@mail.gmail.com>
Date: Tue, 10 Dec 2024 08:28:48 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, hannes@...xchg.org, 
	mjguzik@...il.com, oliver.sang@...el.com, mgorman@...hsingularity.net, 
	david@...hat.com, peterx@...hat.com, oleg@...hat.com, dave@...olabs.net, 
	paulmck@...nel.org, brauner@...nel.org, dhowells@...hat.com, hdanton@...a.com, 
	hughd@...gle.com, minchan@...gle.com, jannh@...gle.com, 
	shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	corbet@....net, linux-doc@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v5 3/6] mm: mark vma as detached until it's added into vma tree

On Tue, Dec 10, 2024 at 3:36 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 12/6/24 23:52, Suren Baghdasaryan wrote:
> > Current implementation does not set detached flag when a VMA is first
> > allocated. This does not represent the real state of the VMA, which is
> > detached until it is added into mm's VMA tree. Fix this by marking new
> > VMAs as detached and resetting detached flag only after VMA is added
> > into a tree.
> > Introduce vma_mark_attached() to make the API more readable and to
> > simplify possible future cleanup when vma->vm_mm might be used to
> > indicate detached vma and vma_mark_attached() will need an additional
> > mm parameter.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 21660a9ad97a..71990f46aa4e 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -465,6 +465,10 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> >       data_race(memcpy(new, orig, sizeof(*new)));
> >       vma_lock_init(new);
> >       INIT_LIST_HEAD(&new->anon_vma_chain);
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +     /* vma is not locked, can't use vma_mark_detached() */
> > +     new->detached = true;
> > +#endif
>
> It occured me we could probably move the detached setting to vma_lock_init()
> to avoid the #ifdef (also in the ctor in next patch)?

Yes but setting vma->detached inside vma_lock_init() does not seem
logical. I went back and forth on this and in the end decided to keep
it this way. If the #ifdef-ery is too ugly I can introduce
vma_init_detached() or something like that.

>
> >       vma_numab_state_init(new);
> >       dup_anon_vma_name(orig, new);
> >