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Message-ID: <20201109173236-mutt-send-email-mst@kernel.org>
Date: Mon, 9 Nov 2020 17:42:57 -0500
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
Subject: Re: [PATCH v2] vhost-vdpa: fix page pinning leakage in error path
(rework)
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 :)
A way to document assumptions is probably BUG_ON.
> >
> > 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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