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

[si-wei liu <si-wei.liu@oracle.com>]

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@oracle.com>
>> ---
>>   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;
>>   }
>

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