Re: [dpdk-dev] [PATCH] net/virtio-user: fix memory hotplug support

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 23-Aug-18 12:19 PM, Sean Harte wrote:
> On Thu, 23 Aug 2018 at 10:05, Burakov, Anatoly
> <anatoly.burakov@intel.com> wrote:
>>
>> On 23-Aug-18 3:57 AM, Tiwei Bie wrote:
>>> Deadlock can occur when allocating memory if a vhost-kernel
>>> based virtio-user device is in use. Besides, it's possible
>>> to have much more than 64 non-contiguous hugepage backed
>>> memory regions due to the memory hotplug, which may cause
>>> problems when handling VHOST_SET_MEM_TABLE request. A better
>>> solution is to have the virtio-user pass all the VA ranges
>>> reserved by DPDK to vhost-kernel.
>>>
>>> Bugzilla ID: 81
>>> Fixes: 12ecb2f63b12 ("net/virtio-user: support memory hotplug")
>>> Cc: stable@dpdk.org
>>>
>>> Reported-by: Seán Harte <seanbh@gmail.com>
>>> Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
>>> ---
>>>    drivers/net/virtio/virtio_user/vhost_kernel.c | 64 ++++++++-----------
>>>    1 file changed, 27 insertions(+), 37 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio/virtio_user/vhost_kernel.c b/drivers/net/virtio/virtio_user/vhost_kernel.c
>>> index b2444096c..49bd1b821 100644
>>> --- a/drivers/net/virtio/virtio_user/vhost_kernel.c
>>> +++ b/drivers/net/virtio/virtio_user/vhost_kernel.c
>>> @@ -70,41 +70,12 @@ static uint64_t vhost_req_user_to_kernel[] = {
>>>        [VHOST_USER_SET_MEM_TABLE] = VHOST_SET_MEM_TABLE,
>>>    };
>>>
>>> -struct walk_arg {
>>> -     struct vhost_memory_kernel *vm;
>>> -     uint32_t region_nr;
>>> -};
>>> -static int
>>> -add_memory_region(const struct rte_memseg_list *msl __rte_unused,
>>> -             const struct rte_memseg *ms, size_t len, void *arg)
>>> -{
>>> -     struct walk_arg *wa = arg;
>>> -     struct vhost_memory_region *mr;
>>> -     void *start_addr;
>>> -
>>> -     if (wa->region_nr >= max_regions)
>>> -             return -1;
>>> -
>>> -     mr = &wa->vm->regions[wa->region_nr++];
>>> -     start_addr = ms->addr;
>>> -
>>> -     mr->guest_phys_addr = (uint64_t)(uintptr_t)start_addr;
>>> -     mr->userspace_addr = (uint64_t)(uintptr_t)start_addr;
>>> -     mr->memory_size = len;
>>> -     mr->mmap_offset = 0;
>>> -
>>> -     return 0;
>>> -}
>>> -
>>> -/* By default, vhost kernel module allows 64 regions, but DPDK allows
>>> - * 256 segments. As a relief, below function merges those virtually
>>> - * adjacent memsegs into one region.
>>> - */
>>>    static struct vhost_memory_kernel *
>>>    prepare_vhost_memory_kernel(void)
>>>    {
>>> +     struct rte_mem_config *mcfg = rte_eal_get_configuration()->mem_config;
>>>        struct vhost_memory_kernel *vm;
>>> -     struct walk_arg wa;
>>> +     uint32_t region_nr = 0, i;
>>>
>>>        vm = malloc(sizeof(struct vhost_memory_kernel) +
>>>                        max_regions *
>>> @@ -112,15 +83,34 @@ prepare_vhost_memory_kernel(void)
>>>        if (!vm)
>>>                return NULL;
>>>
>>> -     wa.region_nr = 0;
>>> -     wa.vm = vm;
>>> +     for (i = 0; i < RTE_MAX_MEMSEG_LISTS; i++) {
>>> +             struct rte_memseg_list *msl = &mcfg->memsegs[i];
>>> +             struct vhost_memory_region *mr;
>>> +             void *start_addr;
>>> +             uint64_t len;
>>
>> There is a rte_memseg_list_walk() - please do not iterate over memseg
>> lists manually.
>>
> 
> rte_memseg_list_walk() can't be used here because
> prepare_vhost_memory_kernel() is sometimes called from a memory
> callback. It will then hang trying to get a read lock on
> memory_hotplug_lock.

OK, so use rte_memseg_list_walk_thread_unsafe().

> I don't think the rte_memseg_list_walk_thread_unsafe() function is
> appropriate because prepare_vhost_memory_kernel() may not always be
> called from a memory callback.

And how is this different? What you're doing here is identical to 
calling rte_memseg_list_walk_thread_unsafe() (that's precisely what it 
does internally - check the code!), except that you're doing it manually 
and not using DPDK API, which makes your code dependent on internals of 
DPDK's memory implementation.

So, this function may or may not be called from a callback, but you're 
using thread-unsafe walk anyway. I think you should call either 
thread-safe or thread-unsafe version depending on whether you're being 
called from a callback or not.

> 
>>>
>>> -     if (rte_memseg_contig_walk(add_memory_region, &wa) < 0) {
>>> -             free(vm);
>>> -             return NULL;
>>> +             start_addr = msl->base_va;
>>> +             len = msl->page_sz * msl->memseg_arr.len;
>>> +
>>> +             if (start_addr == NULL || len == 0)
>>> +                     continue;
>>> +
>>> +             if (region_nr >= max_regions) {
>>> +                     free(vm);
>>> +                     return NULL;
>>> +             }
>>> +
>>> +             mr = &vm->regions[region_nr++];
>>> +             mr->guest_phys_addr = (uint64_t)(uintptr_t)start_addr;
>>> +             mr->userspace_addr = (uint64_t)(uintptr_t)start_addr;
>>> +             mr->memory_size = len;
>>> +             mr->mmap_offset = 0; /* flags_padding */
>>> +
>>> +             PMD_DRV_LOG(DEBUG, "index=%u, addr=%p len=%" PRIu64,
>>> +                             i, start_addr, len);
>>>        }
>>>
>>> -     vm->nregions = wa.region_nr;
>>> +     vm->nregions = region_nr;
>>>        vm->padding = 0;
>>>        return vm;
>>>    }
>>>
>>
>>
>> --
>> Thanks,
>> Anatoly
> 


-- 
Thanks,
Anatoly