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Date: Thu, 6 Apr 2017 11:05:31 -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 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.
Thanks,
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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