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

[Sean Harte <seanbh@gmail.com>]

On Thu, 23 Aug 2018 at 15:01, Burakov, Anatoly
<anatoly.burakov@intel.com> wrote:
>
> 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.
>

I'm not the patch author :-) Switching to the _thread_unsafe()
function might convert the easy-to-debug deadlock issue into a
tricky-to-debug race condition. Picking between the two
implementations depending on the calling context would work. Another
approach might be a different type of locking system that allows
reading when the same thread already holds the write lock.

> >
> >>>
> >>> -     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