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

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Message-ID: <ead9d96f-6cc0-46bb-9f08-585bd1b152e3@suse.cz>
Date: Mon, 13 Jan 2025 21:32:55 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org,
 liam.howlett@...cle.com, david.laight.linux@...il.com, mhocko@...e.com,
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Subject: Re: [PATCH v9 05/17] mm: mark vmas detached upon exit

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.

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.