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Date: Mon, 09 Nov 2020 15:56:21 -0800
From: si-wei liu <si-wei.liu@...cle.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
CC: Jason Wang <jasowang@...hat.com>, 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 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().
> 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.
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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