Re: [PATCH v2] vhost-vdpa: fix page pinning leakage in error path (rework)

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

On 11/4/2020 7:26 PM, Jason Wang wrote:
>
> On 2020/11/5 上午7:33, 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>
>> ---
>> Changes in v2:
>> - Drop the reversion patch
>> - Fix unhandled page leak towards the end of page_list
>>
>>   drivers/vhost/vdpa.c | 79 
>> ++++++++++++++++++++++++++++++++++++++++------------
>>   1 file changed, 61 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
>> index b6d9016..e112854 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 +611,75 @@ 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) {
>> +                    /*
>> +                     * Unpin the pages that are left unmapped
>> +                     * from this point on in the current
>> +                     * page_list. The remaining outstanding
>> +                     * ones which may stride across several
>> +                     * chunks will be covered in the common
>> +                     * error path subsequently.
>> +                     */
>> +                    unpin_user_pages(&page_list[i],
>> +                             pinned - i);
>
>
> Can we simply do last_pfn = this_pfn here?
Nope. They are not contiguous segments of memory. Noted the conditional 
(this_pfn != last_pfn + 1) being held here.

>
>
>>                       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 +687,26 @@ static int 
>> vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
>>                    map_pfn << PAGE_SHIFT, msg->perm);
>>   out:
>>       if (ret) {
>> +        if (nchunks && last_pfn) {
>
>
> Any reason for checking last_pfn here?
>
> Note that we did:
>
> +        nchunks++;
>
>          if (!last_pfn)
>              map_pfn = page_to_pfn(page_list[0]);
It's for explicit coding to make sure this common error path can be 
reused no matter if last_pfn has a sane value assigned or not. I can 
change it to an implicit WARN_ON() if need be.

Thanks,
-Siwei

>
>
> Thanks
>
>
>> +            unsigned long pfn;
>> +
>> +            /*
>> +             * Unpin the outstanding pages which are yet to be
>> +             * mapped but haven't due to vdpa_map() or
>> +             * pin_user_pages() failure.
>> +             *
>> +             * Mapped pages are accounted in vdpa_map(), hence
>> +             * the corresponding unpinning 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);
>> -        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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