From: Florian Westphal <fw@strlen.de>
To: Florian Westphal <fw@strlen.de>
Cc: Michal Hocko <mhocko@suse.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org, vbabka@suse.cz,
akpm@linux-foundation.org, urezki@gmail.com,
netdev@vger.kernel.org, netfilter-devel@vger.kernel.org,
Martin Zaharinov <micron10@gmail.com>
Subject: Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP_ATOMIC
Date: Mon, 26 Sep 2022 15:38:34 +0200 [thread overview]
Message-ID: <20220926133834.GE12777@breakpoint.cc> (raw)
In-Reply-To: <20220926130808.GD12777@breakpoint.cc>
Florian Westphal <fw@strlen.de> wrote:
> Michal Hocko <mhocko@suse.com> wrote:
> > On Mon 26-09-22 12:08:00, Florian Westphal wrote:
> > > Michal Hocko <mhocko@suse.com> wrote:
> > > > + old_tbl = rht_dereference_rcu(ht->tbl, ht);
> > > > + size = tbl->size;
> > > > +
> > > > + data = ERR_PTR(-EBUSY);
> > > > +
> > > > + if (rht_grow_above_75(ht, tbl))
> > > > + size *= 2;
> > > > + /* Do not schedule more than one rehash */
> > > > + else if (old_tbl != tbl)
> > > > + return data;
> > > > +
> > > > + data = ERR_PTR(-ENOMEM);
> > > > +
> > > > + rcu_read_unlock();
> > > > + new_tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
> > > > + rcu_read_lock();
> > >
> > > I don't think this is going to work, there can be callers that
> > > rely on rcu protected data structures getting free'd.
> >
> > The caller of this function drops RCU for each retry, why should be the
> > called function any special?
>
> I was unfortunately never able to fully understand rhashtable.
Obviously.
> AFAICS the rcu_read_lock/unlock in the caller is pointless,
> or at least dubious.
Addedum, I can't read:
void *rhashtable_insert_slow(struct rhashtable *ht, const void *key,
struct rhash_head *obj)
{
void *data;
do {
rcu_read_lock();
data = rhashtable_try_insert(ht, key, obj);
rcu_read_unlock();
}
} while (PTR_ERR(data) == -EAGAIN);
}
... which is needed to prevent a lockdep splat in
rhashtable_try_insert() -- there is no guarantee the caller already
has rcu_read_lock().
> To the best of my knowledge there are users of this interface that
> invoke it with rcu read lock held, and since those always nest, the
> rcu_read_unlock() won't move us to GFP_KERNEL territory.
>
> I guess you can add a might_sleep() and ask kernel to barf at runtime.
I did and it triggers. Caller is inet_frag_find(), triggered
via 'ping -s 60000 $addr'.
next prev parent reply other threads:[~2022-09-26 15:07 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-23 10:38 [PATCH mm] mm: fix BUG with kvzalloc+GFP_ATOMIC Florian Westphal
2022-09-23 13:29 ` Michal Hocko
2022-09-23 13:35 ` Florian Westphal
2022-09-23 14:43 ` Uladzislau Rezki
2022-09-23 14:54 ` Florian Westphal
2022-09-23 15:10 ` Uladzislau Rezki
2022-09-26 15:03 ` Vlastimil Babka
2022-09-26 15:28 ` Sebastian Andrzej Siewior
2022-09-26 7:49 ` Michal Hocko
2022-09-26 7:56 ` Florian Westphal
2022-09-26 8:57 ` Michal Hocko
2022-09-26 9:30 ` Michal Hocko
2022-09-26 10:08 ` Florian Westphal
2022-09-26 12:02 ` Michal Hocko
2022-09-26 13:08 ` Florian Westphal
2022-09-26 13:38 ` Florian Westphal [this message]
2022-09-26 14:05 ` Michal Hocko
2022-09-26 14:20 ` Florian Westphal
2022-09-26 14:32 ` Michal Hocko
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