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Message-ID: <41804db9-27fe-5f5c-9d87-34fd380f3693@redhat.com>
Date: Tue, 10 Nov 2020 11:27:42 +0800
From: Jason Wang <jasowang@...hat.com>
To: si-wei liu <si-wei.liu@...cle.com>,
"Michael S. Tsirkin" <mst@...hat.com>
Cc: lingshan.zhu@...el.com, joao.m.martins@...cle.com,
boris.ostrovsky@...cle.com, linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2] vhost-vdpa: fix page pinning leakage in error path
(rework)
On 2020/11/10 上午7:56, si-wei liu wrote:
>
> On 11/9/2020 2:42 PM, Michael S. Tsirkin wrote:
>> On Mon, Nov 09, 2020 at 01:44:03PM -0800, si-wei liu wrote:
>>> On 11/8/2020 7:21 PM, Jason Wang wrote:
>>>> On 2020/11/6 上午6:57, si-wei liu wrote:
>>>>> 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@...cle.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.
>>>>
>>>> Right.
>>>>
>>>>
>>>>>>
>>>>>>> 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.
>>>>
>>>> Just to make sure I understand. A question, when will we get
>>>> nchunks !=
>>>> 0 but last_pfn == 0?
>>> The current code has implicit assumption that nchunks != 0 infers
>>> last_pfn
>>> != 0. However, this assumption could break subject to code structure
>>> changes
>>> for eg. failure may occur after the increment of nchunks and before
>>> the for
>>> loop. I feel it'd be the best to capture this assumption with something
>>> explicit.
>>>
>>> -Siwei
>> if here isn't really an explicit way to document assumptions,
>> it's a way to avoid assumptions :)
> Agreed. I was referring to the v3 patch which had turned the defensive
> coding to a WARN_ON().
I think I get you now.
>
>> A way to document assumptions is probably BUG_ON.
> If you're fine with below checkpatch warning I can definitely convert
> it to a BUG_ON:
>
> WARNING: Avoid crashing the kernel - try using WARN_ON & recovery code
> rather than BUG() or BUG_ON()
>
> Let me know if I need to post a v4 for this nit.
I will ack for V3.
Thanks
>
> Thanks
> -Siwei
>
>
>
>>
>>>> Thanks
>>>>
>>>>
>>>>> 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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