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From: Yonghong Song <yhs@meta.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Hou Tao <houtao@huaweicloud.com>
Cc: bpf@vger.kernel.org, Martin KaFai Lau <martin.lau@linux.dev>,
	Andrii Nakryiko <andrii@kernel.org>, Song Liu <song@kernel.org>,
	Hao Luo <haoluo@google.com>, Yonghong Song <yhs@fb.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Jiri Olsa <jolsa@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	rcu@vger.kernel.org, houtao1@huawei.com
Subject: Re: [RFC PATCH bpf-next 0/6] bpf: Handle reuse in bpf memory alloc
Date: Sun, 1 Jan 2023 10:48:03 -0800	[thread overview]
Message-ID: <e5f502b5-ea71-8b96-3874-75e0e5a4932f@meta.com> (raw)
In-Reply-To: <20230101012629.nmpofewtlgdutqpe@macbook-pro-6.dhcp.thefacebook.com>



On 12/31/22 5:26 PM, Alexei Starovoitov wrote:
> On Fri, Dec 30, 2022 at 12:11:45PM +0800, Hou Tao wrote:
>> From: Hou Tao <houtao1@huawei.com>
>>
>> Hi,
>>
>> The patchset tries to fix the problems found when checking how htab map
>> handles element reuse in bpf memory allocator. The immediate reuse of
>> freed elements may lead to two problems in htab map:
>>
>> (1) reuse will reinitialize special fields (e.g., bpf_spin_lock) in
>>      htab map value and it may corrupt lookup procedure with BFP_F_LOCK
>>      flag which acquires bpf-spin-lock during value copying. The
>>      corruption of bpf-spin-lock may result in hard lock-up.
>> (2) lookup procedure may get incorrect map value if the found element is
>>      freed and then reused.
>>
>> Because the type of htab map elements are the same, so problem #1 can be
>> fixed by supporting ctor in bpf memory allocator. The ctor initializes
>> these special fields in map element only when the map element is newly
>> allocated. If it is just a reused element, there will be no
>> reinitialization.
> 
> Instead of adding the overhead of ctor callback let's just
> add __GFP_ZERO to flags in __alloc().
> That will address the issue 1 and will make bpf_mem_alloc behave just
> like percpu_freelist, so hashmap with BPF_F_NO_PREALLOC and default
> will behave the same way.

Patch 
https://lore.kernel.org/all/20220809213033.24147-3-memxor@gmail.com/ 
tried to address a similar issue for lru hash table.
Maybe we need to do similar things after bpf_mem_cache_alloc() for
hash table?


> 
>> Problem #2 exists for both non-preallocated and preallocated htab map.
>> By adding seq in htab element, doing reuse check and retrying the
>> lookup procedure may be a feasible solution, but it will make the
>> lookup API being hard to use, because the user needs to check whether
>> the found element is reused or not and repeat the lookup procedure if it
>> is reused. A simpler solution would be just disabling freed elements
>> reuse and freeing these elements after lookup procedure ends.
> 
> You've proposed this 'solution' twice already in qptrie thread and both
> times the answer was 'no, we cannot do this' with reasons explained.
> The 3rd time the answer is still the same.
> This 'issue 2' existed in hashmap since very beginning for many years.
> It's a known quirk. There is nothing to fix really.
> 
> The graph apis (aka new gen data structs) with link list and rbtree are
> in active development. Soon bpf progs will be able to implement their own
> hash maps with explicit bpf_rcu_read_lock. At that time the progs will
> be making the trade off between performance and lookup/delete race.
> So please respin with just __GFP_ZERO and update the patch 6
> to check for lockup only.

  reply	other threads:[~2023-01-01 18:48 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-12-30  4:11 [RFC PATCH bpf-next 0/6] bpf: Handle reuse in bpf memory alloc Hou Tao
2022-12-30  4:11 ` [RFC PATCH bpf-next 1/6] bpf: Support ctor in bpf memory allocator Hou Tao
2022-12-30  4:11 ` [RFC PATCH bpf-next 2/6] bpf: Factor out a common helper free_llist() Hou Tao
2022-12-30  4:11 ` [RFC PATCH bpf-next 3/6] bpf: Pass bitwise flags to bpf_mem_alloc_init() Hou Tao
2022-12-30  4:11 ` [RFC PATCH bpf-next 4/6] bpf: Introduce BPF_MA_NO_REUSE for bpf memory allocator Hou Tao
2022-12-30  4:11 ` [RFC PATCH bpf-next 5/6] bpf: Use BPF_MA_NO_REUSE in htab map Hou Tao
2022-12-30  4:11 ` [RFC PATCH bpf-next 6/6] selftests/bpf: Add test case for element reuse " Hou Tao
2023-01-01  1:26 ` [RFC PATCH bpf-next 0/6] bpf: Handle reuse in bpf memory alloc Alexei Starovoitov
2023-01-01 18:48   ` Yonghong Song [this message]
2023-01-03 13:47     ` Hou Tao
2023-01-04  6:10       ` Yonghong Song
2023-01-04  6:30         ` Hou Tao
2023-01-04  7:14           ` Yonghong Song
2023-01-04 18:26             ` Alexei Starovoitov
2023-02-10 16:32               ` Kumar Kartikeya Dwivedi
2023-02-10 21:06                 ` Alexei Starovoitov
2023-02-11  1:09                   ` Hou Tao
2023-02-11 16:33                     ` Alexei Starovoitov
2023-02-11 16:34                       ` Alexei Starovoitov
2023-02-15  1:54                         ` Martin KaFai Lau
2023-02-15  4:02                           ` Hou Tao
2023-02-15  7:22                             ` Martin KaFai Lau
2023-02-16  2:11                               ` Hou Tao
2023-02-16  7:47                                 ` Martin KaFai Lau
2023-02-16  8:18                                   ` Hou Tao
2023-02-16 13:55                         ` Hou Tao
2023-02-16 16:35                           ` Alexei Starovoitov
2023-02-17  1:19                             ` Hou Tao
2023-02-22 19:30                               ` Alexei Starovoitov
2023-02-15  2:35                       ` Hou Tao
2023-02-15  2:42                         ` Alexei Starovoitov
2023-02-15  3:00                           ` Hou Tao
2023-01-03 13:40   ` Hou Tao
2023-01-03 19:38     ` Alexei Starovoitov
2023-01-10  6:26       ` Martin KaFai Lau

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