linux-kernel - Re: [PATCH v6 7/8] fs/proc/task_mmu: read proc/pid/maps under per-vma lock

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Message-ID: <CAEf4BzYDktFt9R78tQifMrJ7okzA+1LhhiqCi+SpSdq3h4zKyw@mail.gmail.com>
Date: Tue, 15 Jul 2025 10:29:26 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, David Hildenbrand <david@...hat.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, akpm@...ux-foundation.org, 
	peterx@...hat.com, jannh@...gle.com, hannes@...xchg.org, mhocko@...nel.org, 
	paulmck@...nel.org, shuah@...nel.org, adobriyan@...il.com, brauner@...nel.org, 
	josef@...icpanda.com, yebin10@...wei.com, linux@...ssschuh.net, 
	willy@...radead.org, osalvador@...e.de, andrii@...nel.org, 
	ryan.roberts@....com, christophe.leroy@...roup.eu, tjmercier@...gle.com, 
	kaleshsingh@...gle.com, aha310510@...il.com, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 7/8] fs/proc/task_mmu: read proc/pid/maps under per-vma lock

On Tue, Jul 15, 2025 at 10:21 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Tue, Jul 15, 2025 at 06:10:16PM +0100, Lorenzo Stoakes wrote:
> > > For PROCMAP_QUERY, we need priv->mm, but the newly added locked_vma
> > > and locked_vma don't need to be persisted between ioctl calls. So we
> > > can just add those two fields into a small struct, and for seq_file
> > > case have it in priv, but for PROCMAP_QUERY just have it on the stack.
> > > The code can be written to accept this struct to maintain the state,
> > > which for PROCMAP_QUERY ioctl will be very short-lived on the stack
> > > one.
> > >
> > > Would that work?
> >
> > Yeah that's a great idea actually, the stack would obviously give us the
> > per-query invocation thing. Nice!
> >
> > I am kicking myself because I jokingly suggested (off-list) that a helper
> > struct would be the answer to everything (I do love them) and of
> > course... here we are :P
>
> Hm but actually we'd have to invert things I think, what I mean is - since
> these fields can be updated at any time by racing threads, we can't have
> _anything_ in the priv struct that is mutable.
>

Exactly, and I guess I was just being incomplete with just listing two
of the fields that Suren make use of in PROCMAP_QUERY. See below.

> So instead we should do something like:
>
> struct proc_maps_state {
>         const struct proc_maps_private *priv;
>         bool mmap_locked;
>         struct vm_area_struct *locked_vma;
>         struct vma_iterator iter;
>         loff_t last_pos;
> };
>
> static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> {
>         struct seq_file *seq = file->private_data;
>         struct proc_maps_private *priv = seq->private;
>         struct proc_maps_state state = {
>                 .priv = priv,
>         };
>
>         switch (cmd) {
>         case PROCMAP_QUERY:
>                 return do_procmap_query(state, (void __user *)arg);

I guess it's a matter of preference, but I'd actually just pass
seq->priv->mm and arg into do_procmap_query(), which will make it
super obvious that priv is not used or mutated, and all the new stuff
that Suren needs for lockless VMA iteration, including iter (not sure
PROCMAP_QUERY needs last_pos, tbh), I'd just put into this new struct,
which do_procmap_query() can keep private to itself.

Ultimately, I think we are on the same page, it's just a matter of
structuring code and types.

>         default:
>                 return -ENOIOCTLCMD;
>         }
> }
>
> And then we have a stack-based thing with the bits that change and a
> read-only pointer to the bits that must remain static. And we can enforce
> that with const...
>
> We'd have to move the VMI and last_pos out too to make it const.
>
> Anyway the general idea should work!