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: <CAJuCfpEgwdbEXKoMyMFiTHJMV15_g77-7N-m6ykReHLjD9rFLQ@mail.gmail.com>
Date: Wed, 9 Jul 2025 15:06:26 +0000
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, akpm@...ux-foundation.org, 
	Liam.Howlett@...cle.com, david@...hat.com, 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 Wed, Jul 9, 2025 at 3:03 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 7/9/25 16:43, Suren Baghdasaryan wrote:
> > On Wed, Jul 9, 2025 at 1:57 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> On 7/8/25 01:10, Suren Baghdasaryan wrote:
> >> >>> +     rcu_read_unlock();
> >> >>> +     vma = lock_vma_under_mmap_lock(mm, iter, address);
> >> >>> +     rcu_read_lock();
> >> >> OK I guess we hold the RCU lock the whole time as we traverse except when
> >> >> we lock under mmap lock.
> >> > Correct.
> >>
> >> I wonder if it's really necessary? Can't it be done just inside
> >> lock_next_vma()? It would also avoid the unlock/lock dance quoted above.
> >>
> >> Even if we later manage to extend this approach to smaps and employ rcu
> >> locking to traverse the page tables, I'd think it's best to separate and
> >> fine-grain the rcu lock usage for vma iterator and page tables, if only to
> >> avoid too long time under the lock.
> >
> > I thought we would need to be in the same rcu read section while
> > traversing the maple tree using vma_next() but now looking at it,
> > maybe we can indeed enter only while finding and locking the next
> > vma...
> > Liam, would that work? I see struct ma_state containing a node field.
> > Can it be freed from under us if we find a vma, exit rcu read section
> > then re-enter rcu and use the same iterator to find the next vma?
>
> If the rcu protection needs to be contigous, and patch 8 avoids the issue by
> always doing vma_iter_init() after rcu_read_lock() (but does it really avoid
> the issue or is it why we see the syzbot reports?) then I guess in the code
> quoted above we also need a vma_iter_init() after the rcu_read_lock(),
> because although the iterator was used briefly under mmap_lock protection,
> that was then unlocked and there can be a race before the rcu_read_lock().

Quite true. So, let's wait for Liam's confirmation and based on his
answer I'll change the patch by either reducing the rcu read section
or adding the missing vma_iter_init() after we switch to mmap_lock.