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: <3b3521f6-30c8-419e-9615-9228f539251e@suse.cz>
Date: Wed, 9 Jul 2025 17:03:32 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
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 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().