linux-kernel - Re: [PATCH v9 05/17] mm: mark vmas detached upon exit

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Message-ID: <f4bdeef9-4231-449d-b0df-9f6fe6d807b0@lucifer.local>
Date: Tue, 14 Jan 2025 11:36:53 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
        peterz@...radead.org, willy@...radead.org, liam.howlett@...cle.com,
        david.laight.linux@...il.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,
        lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com,
        shakeel.butt@...ux.dev, souravpanda@...gle.com,
        pasha.tatashin@...een.com, klarasmodin@...il.com,
        richard.weiyang@...il.com, corbet@....net, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v9 05/17] mm: mark vmas detached upon exit

On Mon, Jan 13, 2025 at 12:42:53PM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 13, 2025 at 12:32 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> >
> > On 1/13/25 20:11, Suren Baghdasaryan wrote:
> > > On Mon, Jan 13, 2025 at 9:13 AM Lorenzo Stoakes
> > > <lorenzo.stoakes@...cle.com> wrote:
> > >>
> > >> On Mon, Jan 13, 2025 at 09:02:50AM -0800, Suren Baghdasaryan wrote:
> > >> > On Mon, Jan 13, 2025 at 4:05 AM Lorenzo Stoakes
> > >> > <lorenzo.stoakes@...cle.com> wrote:
> > >> > >
> > >> > > On Fri, Jan 10, 2025 at 08:25:52PM -0800, Suren Baghdasaryan wrote:
> > >> > > > When exit_mmap() removes vmas belonging to an exiting task, it does not
> > >> > > > mark them as detached since they can't be reached by other tasks and they
> > >> > > > will be freed shortly. Once we introduce vma reuse, all vmas will have to
> > >> > > > be in detached state before they are freed to ensure vma when reused is
> > >> > > > in a consistent state. Add missing vma_mark_detached() before freeing the
> > >> > > > vma.
> > >> > >
> > >> > > Hmm this really makes me worry that we'll see bugs from this detached
> > >> > > stuff, do we make this assumption anywhere else I wonder?
> > >> >
> > >> > This is the only place which does not currently detach the vma before
> > >> > freeing it. If someone tries adding a case like that in the future,
> > >> > they will be met with vma_assert_detached() inside vm_area_free().
> > >>
> > >> OK good to know!
> > >>
> > >> Again, I wonder if we should make these assertions stronger as commented
> > >> elsewhere, because if we see them in production isn't that worth an actual
> > >> non-debug WARN_ON_ONCE()?
> > >
> > > Sure. I'll change vma_assert_attached()/vma_assert_detached() to use
> > > WARN_ON_ONCE() and to return a bool (see also my reply in the patch
> > > [0/17]).
> >
> > So is this a case of "someone might introduce code later that will violate
> > them" as alluded to above? Unconditional WARN_ON_ONCE seems too much then.

My concern is that there is a broken case that remains hidden because
nothing is actually checked in production, which would then become really
difficult to debug should somebody report it.

We intend the WARN_ONxxx() functions to be asserting things that -should
not be- for precisely this kind of purpose so I think it makes sense here.

>
> Yes, I wanted to make sure refcounting will not be broken by someone
> doing re-attach/re-detach.

Yes, and debugging this without it could be really horrible.

>
> >
> > In general it's not easy to determine how paranoid we should be in non-debug
> > code, but I'm not sure what's the need here specifically.
>
> I'm not sure how strict we should be but we definitely should try to
> catch refcounting mistakes and that's my goal here.

Yes I think it is worth it here (obviously :)

>
> >