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Date: Tue, 03 Nov 2020 18:14:08 -0800
From: si-wei liu <si-wei.liu@...cle.com>
To: Jason Wang <jasowang@...hat.com>, mst@...hat.com,
lingshan.zhu@...el.com
CC: joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/2] vhost-vdpa: fix page pinning leakage in error path
(rework)
On 11/3/2020 5:58 PM, Jason Wang wrote:
>
> On 2020/11/4 上午9:08, si-wei liu wrote:
>>
>> On 11/3/2020 5:06 PM, si-wei liu wrote:
>>>
>>> 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@...cle.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.
>> Typo: ... leaving the vhost_vdpa_process_iotlb_update() 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().
>>>
>
> I see, then I wonder if it would have more easy to read code if we do
> (un)pinning/accouting only in vhost_vdpa_map()/vhost_vdpa_unmap()?
Yes. That's what I've done in my new code. Though, the caller still has
to unpin the outstanding pages that aren't accounted for in
vhost_vdpa_map().
-Siwei
>
> Thanks
>
>
>>>
>>> -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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