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Date: Thu, 6 Apr 2017 13:32:14 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Auger Eric <eric.auger@...hat.com>
Cc: kvm@...r.kernel.org, kwankhede@...dia.com,
linux-kernel@...r.kernel.org, slp@...hat.com
Subject: Re: [PATCH v2] vfio/type1: Remove locked page accounting workqueue
On Thu, 6 Apr 2017 11:05:31 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:
> On Thu, 6 Apr 2017 18:53:04 +0200
> Auger Eric <eric.auger@...hat.com> wrote:
>
> > Hi Alex,
> >
> > On 06/04/2017 16:53, Alex Williamson wrote:
> > > If the mmap_sem is contented then the vfio type1 IOMMU backend will
> > > defer locked page accounting updates to a workqueue task. This has
> > > a few problems and depending on which side the user tries to play,
> > > they might be over-penalized for unmaps that haven't yet been
> > > accounted, or able to race the workqueue to enter more mappings
> > > than they're allowed. It's not entirely clear what motivated this
> > > workqueue mechanism in the original vfio design, but it seems to
> > > introduce more problems than it solves, so remove it and update the
> > > callers to allow for failure. We can also now recheck the limit
> > > under write lock to make sure we don't exceed it.
> > >
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > Looks good to me.
> >
> > Reviewed-by: Eric Auger <eric.auger@...hat.com>
> >
>
> One possible enhancement below.
>
> >
> > > ---
> > >
> > > v2: Fixed missed mmput on failure to acquire mmap_sem as noted by Eric
> > >
> > > drivers/vfio/vfio_iommu_type1.c | 101 ++++++++++++++++++---------------------
> > > 1 file changed, 46 insertions(+), 55 deletions(-)
> > >
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > index 32d2633092a3..b799edbb8c4f 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > @@ -246,69 +246,45 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
> > > return ret;
> > > }
> > >
> > > -struct vwork {
> > > - struct mm_struct *mm;
> > > - long npage;
> > > - struct work_struct work;
> > > -};
> > > -
> > > -/* delayed decrement/increment for locked_vm */
> > > -static void vfio_lock_acct_bg(struct work_struct *work)
> > > +static int vfio_lock_acct(struct task_struct *task, long npage)
> > > {
> > > - struct vwork *vwork = container_of(work, struct vwork, work);
> > > - struct mm_struct *mm;
> > > -
> > > - mm = vwork->mm;
> > > - down_write(&mm->mmap_sem);
> > > - mm->locked_vm += vwork->npage;
> > > - up_write(&mm->mmap_sem);
> > > - mmput(mm);
> > > - kfree(vwork);
> > > -}
> > > -
> > > -static void vfio_lock_acct(struct task_struct *task, long npage)
> > > -{
> > > - struct vwork *vwork;
> > > struct mm_struct *mm;
> > > bool is_current;
> > > + int ret;
> > >
> > > if (!npage)
> > > - return;
> > > + return 0;
> > >
> > > is_current = (task->mm == current->mm);
> > >
> > > mm = is_current ? task->mm : get_task_mm(task);
> > > if (!mm)
> > > - return; /* process exited */
> > > + return -ESRCH; /* process exited */
> > >
> > > - if (down_write_trylock(&mm->mmap_sem)) {
> > > - mm->locked_vm += npage;
> > > - up_write(&mm->mmap_sem);
> > > - if (!is_current)
> > > - mmput(mm);
> > > - return;
> > > - }
> > > + ret = down_write_killable(&mm->mmap_sem);
>
>
> Hmm, since we're already testing current, I wonder if it makes sense to
> have this bimodal, killable if current, straight down_write()
> otherwise. I'm not too sure how important it is to use killable
> regardless, but mlock does, which seemed like a good model to follow.
I see other examples in the kernel where killable is used even for
remote mm, which would match the external page pinning usage, so I think
I'll keep this as is unless there are other comments for v2. Thanks
for the review.
Alex
> > > + if (!ret) {
> > > + if (npage < 0) {
> > > + mm->locked_vm += npage;
> > > + } else {
> > > + unsigned long limit;
> > > +
> > > + limit = is_current ?
> > > + rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT :
> > > + task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > +
> > > + if (mm->locked_vm + npage <= limit)
> > > + mm->locked_vm += npage;
> > > + else
> > > + ret = -ENOMEM;
> > > + }
> > >
> > > - if (is_current) {
> > > - mm = get_task_mm(task);
> > > - if (!mm)
> > > - return;
> > > + up_write(&mm->mmap_sem);
> > > }
> > >
> > > - /*
> > > - * Couldn't get mmap_sem lock, so must setup to update
> > > - * mm->locked_vm later. If locked_vm were atomic, we
> > > - * wouldn't need this silliness
> > > - */
> > > - vwork = kmalloc(sizeof(struct vwork), GFP_KERNEL);
> > > - if (WARN_ON(!vwork)) {
> > > + if (!is_current)
> > > mmput(mm);
> > > - return;
> > > - }
> > > - INIT_WORK(&vwork->work, vfio_lock_acct_bg);
> > > - vwork->mm = mm;
> > > - vwork->npage = npage;
> > > - schedule_work(&vwork->work);
> > > +
> > > + return ret;
> > > }
> > >
> > > /*
> > > @@ -405,7 +381,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> > > static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> > > long npage, unsigned long *pfn_base)
> > > {
> > > - unsigned long limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > + unsigned long pfn = 0, limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > bool lock_cap = capable(CAP_IPC_LOCK);
> > > long ret, pinned = 0, lock_acct = 0;
> > > bool rsvd;
> > > @@ -442,8 +418,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> > > /* Lock all the consecutive pages from pfn_base */
> > > for (vaddr += PAGE_SIZE, iova += PAGE_SIZE; pinned < npage;
> > > pinned++, vaddr += PAGE_SIZE, iova += PAGE_SIZE) {
> > > - unsigned long pfn = 0;
> > > -
> > > ret = vaddr_get_pfn(current->mm, vaddr, dma->prot, &pfn);
> > > if (ret)
> > > break;
> > > @@ -460,14 +434,25 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> > > put_pfn(pfn, dma->prot);
> > > pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
> > > __func__, limit << PAGE_SHIFT);
> > > - break;
> > > + ret = -ENOMEM;
> > > + goto unpin_out;
> > > }
> > > lock_acct++;
> > > }
> > > }
> > >
> > > out:
> > > - vfio_lock_acct(current, lock_acct);
> > > + ret = vfio_lock_acct(current, lock_acct);
> > > +
> > > +unpin_out:
> > > + if (ret) {
> > > + if (!rsvd) {
> > > + for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> > > + put_pfn(pfn, dma->prot);
> > > + }
> > > +
> > > + return ret;
> > > + }
> > >
> > > return pinned;
> > > }
> > > @@ -522,8 +507,14 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> > > goto pin_page_exit;
> > > }
> > >
> > > - if (!rsvd && do_accounting)
> > > - vfio_lock_acct(dma->task, 1);
> > > + if (!rsvd && do_accounting) {
> > > + ret = vfio_lock_acct(dma->task, 1);
> > > + if (ret) {
> > > + put_pfn(*pfn_base, dma->prot);
> > > + goto pin_page_exit;
> > > + }
> > > + }
> > > +
> > > ret = 1;
> > >
> > > pin_page_exit:
> > >
>
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