Re: [PATCH 2/2] vhost-vdpa: fix page pinning leakage in error path (rework)

[si-wei liu]

On 11/3/2020 5:00 AM, Jason Wang wrote:
> On 2020/10/30 下午3:45, Si-Wei Liu wrote:
> > Pinned pages are not properly accounted particularly when
> > mapping error occurs on IOTLB update. Clean up dangling
> > pinned pages for the error path.
> >
> > The memory usage for bookkeeping pinned pages is reverted
> > to what it was before: only one single free page is needed.
> > This helps reduce the host memory demand for VM with a large
> > amount of memory, or in the situation where host is running
> > short of free memory.
> >
> > Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> > Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> > ---
> >   drivers/vhost/vdpa.c | 64 
> > +++++++++++++++++++++++++++++++++++++---------------
> >   1 file changed, 46 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > index b6d9016..8da8558 100644
> > --- a/drivers/vhost/vdpa.c
> > +++ b/drivers/vhost/vdpa.c
> > @@ -560,6 +560,8 @@ static int vhost_vdpa_map(struct vhost_vdpa *v,
> >         if (r)
> >           vhost_iotlb_del_range(dev->iotlb, iova, iova + size - 1);
> > +    else
> > +        atomic64_add(size >> PAGE_SHIFT, &dev->mm->pinned_vm);
> >         return r;
> >   }
> > @@ -591,14 +593,16 @@ static int 
> > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> >       unsigned long list_size = PAGE_SIZE / sizeof(struct page *);
> >       unsigned int gup_flags = FOLL_LONGTERM;
> >       unsigned long npages, cur_base, map_pfn, last_pfn = 0;
> > -    unsigned long locked, lock_limit, pinned, i;
> > +    unsigned long lock_limit, sz2pin, nchunks, i;
> >       u64 iova = msg->iova;
> > +    long pinned;
> >       int ret = 0;
> >         if (vhost_iotlb_itree_first(iotlb, msg->iova,
> >                       msg->iova + msg->size - 1))
> >           return -EEXIST;
> >   +    /* Limit the use of memory for bookkeeping */
> >       page_list = (struct page **) __get_free_page(GFP_KERNEL);
> >       if (!page_list)
> >           return -ENOMEM;
> > @@ -607,52 +611,64 @@ static int 
> > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> >           gup_flags |= FOLL_WRITE;
> >         npages = PAGE_ALIGN(msg->size + (iova & ~PAGE_MASK)) >> 
> > PAGE_SHIFT;
> > -    if (!npages)
> > -        return -EINVAL;
> > +    if (!npages) {
> > +        ret = -EINVAL;
> > +        goto free;
> > +    }
> >         mmap_read_lock(dev->mm);
> >   -    locked = atomic64_add_return(npages, &dev->mm->pinned_vm);
> >       lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > -
> > -    if (locked > lock_limit) {
> > +    if (npages + atomic64_read(&dev->mm->pinned_vm) > lock_limit) {
> >           ret = -ENOMEM;
> > -        goto out;
> > +        goto unlock;
> >       }
> >         cur_base = msg->uaddr & PAGE_MASK;
> >       iova &= PAGE_MASK;
> > +    nchunks = 0;
> >         while (npages) {
> > -        pinned = min_t(unsigned long, npages, list_size);
> > -        ret = pin_user_pages(cur_base, pinned,
> > -                     gup_flags, page_list, NULL);
> > -        if (ret != pinned)
> > +        sz2pin = min_t(unsigned long, npages, list_size);
> > +        pinned = pin_user_pages(cur_base, sz2pin,
> > +                    gup_flags, page_list, NULL);
> > +        if (sz2pin != pinned) {
> > +            if (pinned < 0) {
> > +                ret = pinned;
> > +            } else {
> > +                unpin_user_pages(page_list, pinned);
> > +                ret = -ENOMEM;
> > +            }
> >               goto out;
> > +        }
> > +        nchunks++;
> >             if (!last_pfn)
> >               map_pfn = page_to_pfn(page_list[0]);
> >   -        for (i = 0; i < ret; i++) {
> > +        for (i = 0; i < pinned; i++) {
> >               unsigned long this_pfn = page_to_pfn(page_list[i]);
> >               u64 csize;
> >                 if (last_pfn && (this_pfn != last_pfn + 1)) {
> >                   /* Pin a contiguous chunk of memory */
> >                   csize = (last_pfn - map_pfn + 1) << PAGE_SHIFT;
> > -                if (vhost_vdpa_map(v, iova, csize,
> > -                           map_pfn << PAGE_SHIFT,
> > -                           msg->perm))
> > +                ret = vhost_vdpa_map(v, iova, csize,
> > +                             map_pfn << PAGE_SHIFT,
> > +                             msg->perm);
> > +                if (ret)
> >                       goto out;
> > +
> >                   map_pfn = this_pfn;
> >                   iova += csize;
> > +                nchunks = 0;
> >               }
> >                 last_pfn = this_pfn;
> >           }
> >   -        cur_base += ret << PAGE_SHIFT;
> > -        npages -= ret;
> > +        cur_base += pinned << PAGE_SHIFT;
> > +        npages -= pinned;
> >       }
> >         /* Pin the rest chunk */
> > @@ -660,10 +676,22 @@ static int 
> > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> >                    map_pfn << PAGE_SHIFT, msg->perm);
> >   out:
> >       if (ret) {
> > +        if (nchunks && last_pfn) {
> > +            unsigned long pfn;
> > +
> > +            /*
> > +             * Unpin the outstanding pages which are unmapped.
> > +             * Mapped pages are accounted in vdpa_map(), thus
> > +             * will be handled by vdpa_unmap().
> > +             */
> > +            for (pfn = map_pfn; pfn <= last_pfn; pfn++)
> > +                unpin_user_page(pfn_to_page(pfn));
> > +        }
> >           vhost_vdpa_unmap(v, msg->iova, msg->size);
>
>
> I want to know what's wrong with current code.
>
> We call vhost_vdpa_unmap() on error which calls 
> vhost_vdpa_iotlb_unmap() that will unpin and reduce the pinned_vm.
Think about the case where vhost_vdpa_map() fails in the middle after 
making a few successful ones. In the current code, the 
vhost_vdpa_iotlb_unmap() unpins what had been mapped, but does not unpin 
those that have not yet been mapped. These outstanding pinned pages will 
be leaked after leaving the vhost_vdpa_map() function.

Also, the subtraction accounting at the end of the function is incorrect 
in that case: @npages is deduced by @pinned in each iteration. That's 
why I moved the accounting to vhost_vdpa_map() to be symmetric with 
vhost_vdpa_unmap().


-Siwei
> Thanks
>
>
> > -        atomic64_sub(npages, &dev->mm->pinned_vm);
> >       }
> > +unlock:
> >       mmap_read_unlock(dev->mm);
> > +free:
> >       free_page((unsigned long)page_list);
> >       return ret;
> >   }