WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342
Alexei Starovoitov
alexei.starovoitov at gmail.com
Tue Aug 29 08:26:08 PDT 2023
On Mon, Aug 28, 2023 at 6:57 AM Hou Tao <houtao at huaweicloud.com> wrote:
>
> Hi,
>
> On 8/27/2023 10:53 PM, Yonghong Song wrote:
> >
> >
> > On 8/27/23 1:37 AM, Björn Töpel wrote:
> >> Björn Töpel <bjorn at kernel.org> writes:
> >>
> >>> Hou Tao <houtao at huaweicloud.com> writes:
> >>>
> >>>> Hi,
> >>>>
> >>>> On 8/26/2023 5:23 PM, Björn Töpel wrote:
> >>>>> Hou Tao <houtao at huaweicloud.com> writes:
> >>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> On 8/25/2023 11:28 PM, Yonghong Song wrote:
> >>>>>>>
> >>>>>>> On 8/25/23 3:32 AM, Björn Töpel wrote:
> >>>>>>>> I'm chasing a workqueue hang on RISC-V/qemu (TCG), using the bpf
> >>>>>>>> selftests on bpf-next 9e3b47abeb8f.
> >>>>>>>>
> >>>>>>>> I'm able to reproduce the hang by multiple runs of:
> >>>>>>>> | ./test_progs -a link_api -a linked_list
> >>>>>>>> I'm currently investigating that.
> >>>>>>>>
> >>>>>>>> But! Sometimes (every blue moon) I get a warn_on_once hit:
> >>>>>>>> | ------------[ cut here ]------------
> >>>>>>>> | WARNING: CPU: 3 PID: 261 at kernel/bpf/memalloc.c:342
> >>>>>>>> bpf_mem_refill+0x1fc/0x206
> >>>>>>>> | Modules linked in: bpf_testmod(OE)
> >>>>>>>> | CPU: 3 PID: 261 Comm: test_progs-cpuv Tainted: G OE
> >>>>>>>> N 6.5.0-rc5-01743-gdcb152bb8328 #2
> >>>>>>>> | Hardware name: riscv-virtio,qemu (DT)
> >>>>>>>> | epc : bpf_mem_refill+0x1fc/0x206
> >>>>>>>> | ra : irq_work_single+0x68/0x70
> >>>>>>>> | epc : ffffffff801b1bc4 ra : ffffffff8015fe84 sp :
> >>>>>>>> ff2000000001be20
> >>>>>>>> | gp : ffffffff82d26138 tp : ff6000008477a800 t0 :
> >>>>>>>> 0000000000046600
> >>>>>>>> | t1 : ffffffff812b6ddc t2 : 0000000000000000 s0 :
> >>>>>>>> ff2000000001be70
> >>>>>>>> | s1 : ff5ffffffffe8998 a0 : ff5ffffffffe8998 a1 :
> >>>>>>>> ff600003fef4b000
> >>>>>>>> | a2 : 000000000000003f a3 : ffffffff80008250 a4 :
> >>>>>>>> 0000000000000060
> >>>>>>>> | a5 : 0000000000000080 a6 : 0000000000000000 a7 :
> >>>>>>>> 0000000000735049
> >>>>>>>> | s2 : ff5ffffffffe8998 s3 : 0000000000000022 s4 :
> >>>>>>>> 0000000000001000
> >>>>>>>> | s5 : 0000000000000007 s6 : ff5ffffffffe8570 s7 :
> >>>>>>>> ffffffff82d6bd30
> >>>>>>>> | s8 : 000000000000003f s9 : ffffffff82d2c5e8 s10:
> >>>>>>>> 000000000000ffff
> >>>>>>>> | s11: ffffffff82d2c5d8 t3 : ffffffff81ea8f28 t4 :
> >>>>>>>> 0000000000000000
> >>>>>>>> | t5 : ff6000008fd28278 t6 : 0000000000040000
> >>>>>>>> | status: 0000000200000100 badaddr: 0000000000000000 cause:
> >>>>>>>> 0000000000000003
> >>>>>>>> | [<ffffffff801b1bc4>] bpf_mem_refill+0x1fc/0x206
> >>>>>>>> | [<ffffffff8015fe84>] irq_work_single+0x68/0x70
> >>>>>>>> | [<ffffffff8015feb4>] irq_work_run_list+0x28/0x36
> >>>>>>>> | [<ffffffff8015fefa>] irq_work_run+0x38/0x66
> >>>>>>>> | [<ffffffff8000828a>] handle_IPI+0x3a/0xb4
> >>>>>>>> | [<ffffffff800a5c3a>] handle_percpu_devid_irq+0xa4/0x1f8
> >>>>>>>> | [<ffffffff8009fafa>] generic_handle_domain_irq+0x28/0x36
> >>>>>>>> | [<ffffffff800ae570>] ipi_mux_process+0xac/0xfa
> >>>>>>>> | [<ffffffff8000a8ea>] sbi_ipi_handle+0x2e/0x88
> >>>>>>>> | [<ffffffff8009fafa>] generic_handle_domain_irq+0x28/0x36
> >>>>>>>> | [<ffffffff807ee70e>] riscv_intc_irq+0x36/0x4e
> >>>>>>>> | [<ffffffff812b5d3a>] handle_riscv_irq+0x54/0x86
> >>>>>>>> | [<ffffffff812b6904>] do_irq+0x66/0x98
> >>>>>>>> | ---[ end trace 0000000000000000 ]---
> >>>>>>>>
> >>>>>>>> Code:
> >>>>>>>> | static void free_bulk(struct bpf_mem_cache *c)
> >>>>>>>> | {
> >>>>>>>> | struct bpf_mem_cache *tgt = c->tgt;
> >>>>>>>> | struct llist_node *llnode, *t;
> >>>>>>>> | unsigned long flags;
> >>>>>>>> | int cnt;
> >>>>>>>> |
> >>>>>>>> | WARN_ON_ONCE(tgt->unit_size != c->unit_size);
> >>>>>>>> | ...
> >>>>>>>>
> >>>>>>>> I'm not well versed in the memory allocator; Before I dive into
> >>>>>>>> it --
> >>>>>>>> has anyone else hit it? Ideas on why the warn_on_once is hit?
> >>>>>>> Maybe take a look at the patch
> >>>>>>> 822fb26bdb55 bpf: Add a hint to allocated objects.
> >>>>>>>
> >>>>>>> In the above patch, we have
> >>>>>>>
> >>>>>>> + /*
> >>>>>>> + * Remember bpf_mem_cache that allocated this object.
> >>>>>>> + * The hint is not accurate.
> >>>>>>> + */
> >>>>>>> + c->tgt = *(struct bpf_mem_cache **)llnode;
> >>>>>>>
> >>>>>>> I suspect that the warning may be related to the above.
> >>>>>>> I tried the above ./test_progs command line (running multiple
> >>>>>>> at the same time) and didn't trigger the issue.
> >>>>>> The extra 8-bytes before the freed pointer is used to save the
> >>>>>> pointer
> >>>>>> of the original bpf memory allocator where the freed pointer came
> >>>>>> from,
> >>>>>> so unit_free() could free the pointer back to the original
> >>>>>> allocator to
> >>>>>> prevent alloc-and-free unbalance.
> >>>>>>
> >>>>>> I suspect that a wrong pointer was passed to bpf_obj_drop, but do
> >>>>>> not
> >>>>>> find anything suspicious after checking linked_list. Another
> >>>>>> possibility
> >>>>>> is that there is write-after-free problem which corrupts the extra
> >>>>>> 8-bytes before the freed pointer. Could you please apply the
> >>>>>> following
> >>>>>> debug patch to check whether or not the extra 8-bytes are
> >>>>>> corrupted ?
> >>>>> Thanks for getting back!
> >>>>>
> >>>>> I took your patch for a run, and there's a hit:
> >>>>> | bad cache ff5ffffffffe8570: got size 96 work
> >>>>> ffffffff801b19c8, cache ff5ffffffffe8980 exp size 128 work
> >>>>> ffffffff801b19c8
> >>>>
> >>>> The extra 8-bytes are not corrupted. Both of these two
> >>>> bpf_mem_cache are
> >>>> valid and there are in the cache array defined in bpf_mem_caches. BPF
> >>>> memory allocator allocated the pointer from 96-bytes sized-cache,
> >>>> but it
> >>>> tried to free the pointer through 128-bytes sized-cache.
> >>>>
> >>>> Now I suspect there is no 96-bytes slab in your system and ksize(ptr -
> >>>> LLIST_NODE_SZ) returns 128, so bpf memory allocator selected the
> >>>> 128-byte sized-cache instead of 96-bytes sized-cache. Could you please
> >>>> check the value of KMALLOC_MIN_SIZE in your kernel .config and
> >>>> using the
> >>>> following command to check whether there is 96-bytes slab in your
> >>>> system:
> >>>
> >>> KMALLOC_MIN_SIZE is 64.
> >>>
> >>>> $ cat /proc/slabinfo |grep kmalloc-96
> >>>> dma-kmalloc-96 0 0 96 42 1 : tunables 0 0
> >>>> 0 : slabdata 0 0 0
> >>>> kmalloc-96 1865 2268 96 42 1 : tunables 0 0
> >>>> 0 : slabdata 54 54 0
> >>>>
> >>>> In my system, slab has 96-bytes cached, so grep outputs something,
> >>>> but I
> >>>> think there will no output in your system.
> >>>
> >>> You're right! No kmalloc-96.
> >>
> >> To get rid of the warning, limit available sizes from
> >> bpf_mem_alloc_init()?
>
> It is not enough. We need to adjust size_index accordingly during
> initialization. Could you please try the attached patch below ? It is
> not a formal patch and I am considering to disable prefilling for these
> redirected bpf_mem_caches.
> >
> > Do you know why your system does not have kmalloc-96?
>
> According to the implementation of setup_kmalloc_cache_index_table() and
> create_kmalloc_caches(), when KMALLOC_MIN_SIZE is greater than 64,
> kmalloc-96 will be omitted. If KMALLOC_MIN_SIZE is greater than 128,
> kmalloc-192 will be omitted as well.
Great catch. The fix looks good.
Please submit it officially and add an error check to bpf_mem_alloc_init()
that verifies that ksize() matches the expectations.
The alternative is to use kmalloc_size_roundup() during alloc for
checking instead of ksize().
Technically we can use kmalloc_size_roundup in unit_alloc() and avoid
setup_kmalloc_cache_index_table()-like copy paste, but performance
overhead might be too high.
So your patch + error check at bpf_mem_alloc_init() is preferred.
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