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Date: Wed, 14 Oct 2020 02:52:13 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: si-wei liu <si-wei.liu@...cle.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, netdev@...r.kernel.org
Subject: Re: [PATCH v3 2/2] vhost-vdpa: fix page pinning leakage in error path
On Tue, Oct 13, 2020 at 04:42:59PM -0700, si-wei liu wrote:
>
> On 10/9/2020 7:27 PM, Jason Wang wrote:
> >
> > On 2020/10/3 下午1:02, 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. As the inflight pinned
> > > pages, specifically for memory region that strides across
> > > multiple chunks, would need more than one free page for
> > > book keeping and accounting. For simplicity, pin pages
> > > for all memory in the IOVA range in one go rather than
> > > have multiple pin_user_pages calls to make up the entire
> > > region. This way it's easier to track and account the
> > > pages already mapped, particularly for clean-up in the
> > > error path.
> > >
> > > Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> > > Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
> > > ---
> > > Changes in v3:
> > > - Factor out vhost_vdpa_map() change to a separate patch
> > >
> > > Changes in v2:
> > > - Fix incorrect target SHA1 referenced
> > >
> > > drivers/vhost/vdpa.c | 119
> > > ++++++++++++++++++++++++++++++---------------------
> > > 1 file changed, 71 insertions(+), 48 deletions(-)
> > >
> > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > > index 0f27919..dad41dae 100644
> > > --- a/drivers/vhost/vdpa.c
> > > +++ b/drivers/vhost/vdpa.c
> > > @@ -595,21 +595,19 @@ static int
> > > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> > > struct vhost_dev *dev = &v->vdev;
> > > struct vhost_iotlb *iotlb = dev->iotlb;
> > > struct page **page_list;
> > > - unsigned long list_size = PAGE_SIZE / sizeof(struct page *);
> > > + struct vm_area_struct **vmas;
> > > 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 map_pfn, last_pfn = 0;
> > > + unsigned long npages, lock_limit;
> > > + unsigned long i, nmap = 0;
> > > u64 iova = msg->iova;
> > > + long pinned;
> > > int ret = 0;
> > > if (vhost_iotlb_itree_first(iotlb, msg->iova,
> > > msg->iova + msg->size - 1))
> > > return -EEXIST;
> > > - page_list = (struct page **) __get_free_page(GFP_KERNEL);
> > > - if (!page_list)
> > > - return -ENOMEM;
> > > -
> > > if (msg->perm & VHOST_ACCESS_WO)
> > > gup_flags |= FOLL_WRITE;
> > > @@ -617,61 +615,86 @@ static int
> > > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> > > if (!npages)
> > > return -EINVAL;
> > > + page_list = kvmalloc_array(npages, sizeof(struct page *),
> > > GFP_KERNEL);
> > > + vmas = kvmalloc_array(npages, sizeof(struct vm_area_struct *),
> > > + GFP_KERNEL);
> >
> >
> > This will result high order memory allocation which was what the code
> > tried to avoid originally.
> >
> > Using an unlimited size will cause a lot of side effects consider VM or
> > userspace may try to pin several TB of memory.
> Hmmm, that's a good point. Indeed, if the guest memory demand is huge or the
> host system is running short of free pages, kvmalloc will be problematic and
> less efficient than the __get_free_page implementation.
OK so ... Jason, what's the plan?
How about you send a patchset with
1. revert this change
2. fix error handling leak
> >
> >
> > > + if (!page_list || !vmas) {
> > > + ret = -ENOMEM;
> > > + goto free;
> > > + }
> >
> >
> > Any reason that you want to use vmas?
> Without providing custom vmas, it's subject to high order allocation
> failure. While page_list and vmas can now fallback to virtual memory
> allocation if need be.
>
> >
> >
> > > +
> > > 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;
> > > + pinned = pin_user_pages(msg->uaddr & PAGE_MASK, npages, gup_flags,
> > > + page_list, vmas);
> > > + if (npages != pinned) {
> > > + if (pinned < 0) {
> > > + ret = pinned;
> > > + } else {
> > > + unpin_user_pages(page_list, pinned);
> > > + ret = -ENOMEM;
> > > + }
> > > + goto unlock;
> > > + }
> > > - 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)
> > > - goto out;
> > > -
> > > - if (!last_pfn)
> > > - map_pfn = page_to_pfn(page_list[0]);
> > > -
> > > - for (i = 0; i < ret; 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))
> > > - goto out;
> > > - map_pfn = this_pfn;
> > > - iova += csize;
> > > + iova &= PAGE_MASK;
> > > + map_pfn = page_to_pfn(page_list[0]);
> > > +
> > > + /* One more iteration to avoid extra vdpa_map() call out of
> > > loop. */
> > > + for (i = 0; i <= npages; i++) {
> > > + unsigned long this_pfn;
> > > + u64 csize;
> > > +
> > > + /* The last chunk may have no valid PFN next to it */
> > > + this_pfn = i < npages ? page_to_pfn(page_list[i]) : -1UL;
> > > +
> > > + if (last_pfn && (this_pfn == -1UL ||
> > > + this_pfn != last_pfn + 1)) {
> > > + /* Pin a contiguous chunk of memory */
> > > + csize = last_pfn - map_pfn + 1;
> > > + ret = vhost_vdpa_map(v, iova, csize << PAGE_SHIFT,
> > > + map_pfn << PAGE_SHIFT,
> > > + msg->perm);
> > > + if (ret) {
> > > + /*
> > > + * Unpin the rest chunks of memory on the
> > > + * flight with no corresponding vdpa_map()
> > > + * calls having been made yet. On the other
> > > + * hand, vdpa_unmap() in the failure path
> > > + * is in charge of accounting the number of
> > > + * pinned pages for its own.
> > > + * This asymmetrical pattern of accounting
> > > + * is for efficiency to pin all pages at
> > > + * once, while there is no other callsite
> > > + * of vdpa_map() than here above.
> > > + */
> > > + unpin_user_pages(&page_list[nmap],
> > > + npages - nmap);
> > > + goto out;
> > > }
> > > -
> > > - last_pfn = this_pfn;
> > > + atomic64_add(csize, &dev->mm->pinned_vm);
> > > + nmap += csize;
> > > + iova += csize << PAGE_SHIFT;
> > > + map_pfn = this_pfn;
> > > }
> > > -
> > > - cur_base += ret << PAGE_SHIFT;
> > > - npages -= ret;
> > > + last_pfn = this_pfn;
> > > }
> >
> >
> > So what I suggest is to fix the pinning leakage first and do the
> > possible optimization on top (which is still questionable to me).
> OK. Unfortunately, this was picked and got merged in upstream. So I will
> post a follow up patch set to 1) revert the commit to the original
> __get_free_page() implementation, and 2) fix the accounting and leakage on
> top. Will it be fine?
>
>
> -Siwei
> >
> > Thanks
> >
> >
> > > - /* Pin the rest chunk */
> > > - ret = vhost_vdpa_map(v, iova, (last_pfn - map_pfn + 1) <<
> > > PAGE_SHIFT,
> > > - map_pfn << PAGE_SHIFT, msg->perm);
> > > + WARN_ON(nmap != npages);
> > > out:
> > > - if (ret) {
> > > + if (ret)
> > > vhost_vdpa_unmap(v, msg->iova, msg->size);
> > > - atomic64_sub(npages, &dev->mm->pinned_vm);
> > > - }
> > > +unlock:
> > > mmap_read_unlock(dev->mm);
> > > - free_page((unsigned long)page_list);
> > > +free:
> > > + kvfree(vmas);
> > > + kvfree(page_list);
> > > return ret;
> > > }
> >
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