From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Brian Vazquez <brianvv.kernel@gmail.com>
Cc: Brian Vazquez <brianvv@google.com>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
"David S . Miller" <davem@davemloft.net>,
Stanislav Fomichev <sdf@google.com>,
Willem de Bruijn <willemb@google.com>,
Petar Penkov <ppenkov@google.com>,
LKML <linux-kernel@vger.kernel.org>,
Network Development <netdev@vger.kernel.org>,
bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call
Date: Wed, 24 Jul 2019 18:33:20 -0400 [thread overview]
Message-ID: <CAF=yD-L6RpnxptBtcpVGzP4UoPLRxr2JiQGyRCoTca4jHioPXw@mail.gmail.com> (raw)
In-Reply-To: <CABCgpaWCLJtDx8kHNiQZneqYZkZ3fzRGnipT5__kmwMhu01g=w@mail.gmail.com>
> > > Because maps can be called from userspace and kernel code, this function
> > > can have a scenario where the next_key was found but by the time we
> > > try to retrieve the value the element is not there, in this case the
> > > function continues and tries to get a new next_key value, skipping the
> > > deleted key. If at some point the function find itself trap in a loop,
> > > it will return -EINTR.
> >
> > Good to point this out! I don't think that unbounded continue;
> > statements until an interrupt happens is sufficient. Please bound the
> > number of retries to a low number.
>
> And what would it be a good number? Maybe 3 attempts?
3 sounds good to me.
> And in that case what error should be reported?
One that's unambiguous and somewhat intuitive for the given issue.
Perhaps EBUSY?
> > > The function will try to fit as much as possible in the buf provided and
> > > will return -EINVAL if buf_len is smaller than elem_size.
> > >
> > > QUEUE and STACK maps are not supported.
> > >
> > > Note that map_dump doesn't guarantee that reading the entire table is
> > > consistent since this function is always racing with kernel and user code
> > > but the same behaviour is found when the entire table is walked using
> > > the current interfaces: map_get_next_key + map_lookup_elem.
> >
> > > It is also important to note that with a locked map, the lock is grabbed
> > > for 1 entry at the time, meaning that the returned buf might or might not
> > > be consistent.
> >
> > Would it be informative to signal to the caller if the read was
> > complete and consistent (because the entire table was read while the
> > lock was held)?
>
> Mmm.. not sure how we could signal that to the caller. But I don't
> think there's a way to know it was consistent
Okay, that makes for a simple answer :) No need to try to add a signal, then.
next prev parent reply other threads:[~2019-07-24 22:34 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-24 16:57 [PATCH bpf-next 0/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call Brian Vazquez
2019-07-24 16:57 ` [PATCH bpf-next 1/6] bpf: add bpf_map_value_size and bp_map_copy_value helper functions Brian Vazquez
2019-07-24 20:53 ` Song Liu
2019-07-24 16:57 ` [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call Brian Vazquez
2019-07-24 19:54 ` Willem de Bruijn
2019-07-24 22:26 ` Brian Vazquez
2019-07-24 22:33 ` Willem de Bruijn [this message]
2019-07-24 21:40 ` Song Liu
2019-07-24 22:44 ` Brian Vazquez
2019-07-24 23:04 ` Song Liu
2019-07-25 23:25 ` Brian Vazquez
2019-07-25 23:54 ` Alexei Starovoitov
2019-07-26 1:02 ` Willem de Bruijn
2019-07-26 1:24 ` Brian Vazquez
2019-07-26 1:47 ` Alexei Starovoitov
2019-07-26 6:10 ` Yonghong Song
2019-07-26 23:36 ` Brian Vazquez
2019-07-27 0:02 ` Jakub Kicinski
2019-07-27 17:54 ` Yonghong Song
2019-07-24 16:58 ` [PATCH bpf-next 3/6] bpf: keep bpf.h in sync with tools/ Brian Vazquez
2019-07-24 21:41 ` Song Liu
2019-07-24 23:10 ` Andrii Nakryiko
2019-07-25 23:27 ` Brian Vazquez
2019-07-24 16:58 ` [PATCH bpf-next 4/6] libbpf: support BPF_MAP_DUMP command Brian Vazquez
2019-07-24 19:51 ` Willem de Bruijn
2019-07-24 16:58 ` [PATCH bpf-next 5/6] selftests/bpf: test BPF_MAP_DUMP command on a bpf hashmap Brian Vazquez
2019-07-24 21:58 ` Song Liu
2019-07-24 16:58 ` [PATCH bpf-next 6/6] selftests/bpf: add test to measure performance of BPF_MAP_DUMP Brian Vazquez
2019-07-24 22:01 ` Song Liu
2019-07-24 19:20 ` [PATCH bpf-next 0/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call Song Liu
2019-07-24 22:15 ` Brian Vazquez
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