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Date: Sat, 15 Apr 2017 00:51:28 +0530
From: Kirti Wankhede <kwankhede@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>, <kvm@...r.kernel.org>
CC: <eric.auger@...hat.com>, <peterx@...hat.com>,
<linux-kernel@...r.kernel.org>, <slp@...hat.com>
Subject: Re: [PATCH v3] vfio/type1: Remove locked page accounting workqueue
On 4/12/2017 12:58 AM, 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
> race the workqueue to enter more mappings than they're allowed. The
> original intent of this workqueue mechanism seems to be focused on
> reducing latency through the ioctl, but we cannot do so at the cost
> of correctness. Remove this workqueue mechanism 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.
>
> vfio_pin_pages_remote() also now necessarily includes an unwind path
> which we can jump to directly if the consecutive page pinning finds
> that we're exceeding the user's memory limits. This avoids the
> current lazy approach which does accounting and mapping up to the
> fault, only to return an error on the next iteration to unwind the
> entire vfio_dma.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>
> v3: Update for comments from Peter
> - Use task_rlimit() exclusively
> - Discuss vfio_pin_pages_remote() exit branch in commitlog
>
> drivers/vfio/vfio_iommu_type1.c | 99 +++++++++++++++++----------------------
> 1 file changed, 44 insertions(+), 55 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 32d2633092a3..176ebcc0ffa2 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -246,69 +246,43 @@ 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)
> -{
> - 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)
> +static int 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);
> + if (!ret) {
> + if (npage < 0) {
> + mm->locked_vm += npage;
> + } else {
> + unsigned long limit;
>
> - if (is_current) {
> - mm = get_task_mm(task);
> - if (!mm)
> - return;
> + limit = task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +
> + if (mm->locked_vm + npage <= limit)
> + mm->locked_vm += npage;
> + else
> + ret = -ENOMEM;
> + }
> +
Sorry if I'm late here on my review.
There are rlimit checks before calling vfio_lock_acct() while pinning
pages. I agree this is checked holding locks, so this check is more
robust, but still it feels redundant. I think you can remove checks from
vfio_pin_page_external() and vfio_pin_pages_remote().
Also while checking the limit, !lock_cap checks is not considered here.
That would mean that there code would impose limit check even without
lock capability?
Thanks,
Kirti
> + 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 +379,7 @@ 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 +416,6 @@ 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 +432,25 @@ 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 +505,14 @@ 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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