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Date: Mon, 22 May 2017 10:15:43 +0200
From: Auger Eric <eric.auger@...hat.com>
To: Alex Williamson <alex.williamson@...hat.com>,
linux-kernel@...r.kernel.org
Cc: Kirti Wankhede <kwankhede@...dia.com>, stable@...r.kernel.org,
Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v4.9] vfio/type1: Remove locked page accounting workqueue
Hi,
On 16/05/2017 21:52, Alex Williamson wrote:
> commit 0cfef2b7410b64d7a430947e0b533314c4f97153 upstream.
>
> 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
> Reviewed-by: Peter Xu <peterx@...hat.com>
> Reviewed-by: Kirti Wankhede <kwankhede@...dia.com>
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
Reviewed-by: Eric Auger <eric.auger@...hat.com>
Eric
> ---
>
> Proposed backport for kernels v4.7..v4.9 (pre-mdev, with down_write_killable)
>
> drivers/vfio/vfio_iommu_type1.c | 102 +++++++++++++++++----------------------
> 1 file changed, 44 insertions(+), 58 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 2ba19424e4a1..1d48e62f4f52 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -130,57 +130,36 @@ static void vfio_unlink_dma(struct vfio_iommu *iommu, struct vfio_dma *old)
> rb_erase(&old->node, &iommu->dma_list);
> }
>
> -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(long npage, bool *lock_cap)
> {
> - 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);
> -}
> + int ret;
>
> -static void vfio_lock_acct(long npage)
> -{
> - struct vwork *vwork;
> - struct mm_struct *mm;
> + if (!npage)
> + return 0;
>
> - if (!current->mm || !npage)
> - return; /* process exited or nothing to do */
> + if (!current->mm)
> + return -ESRCH; /* process exited */
> +
> + ret = down_write_killable(¤t->mm->mmap_sem);
> + if (!ret) {
> + if (npage > 0) {
> + if (lock_cap ? !*lock_cap : !capable(CAP_IPC_LOCK)) {
> + unsigned long limit;
> +
> + limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +
> + if (current->mm->locked_vm + npage > limit)
> + ret = -ENOMEM;
> + }
> + }
> +
> + if (!ret)
> + current->mm->locked_vm += npage;
>
> - if (down_write_trylock(¤t->mm->mmap_sem)) {
> - current->mm->locked_vm += npage;
> up_write(¤t->mm->mmap_sem);
> - return;
> }
>
> - /*
> - * 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 (!vwork)
> - return;
> - mm = get_task_mm(current);
> - if (!mm) {
> - kfree(vwork);
> - return;
> - }
> - INIT_WORK(&vwork->work, vfio_lock_acct_bg);
> - vwork->mm = mm;
> - vwork->npage = npage;
> - schedule_work(&vwork->work);
> + return ret;
> }
>
> /*
> @@ -262,9 +241,9 @@ static int vaddr_get_pfn(unsigned long vaddr, int prot, unsigned long *pfn)
> static long vfio_pin_pages(unsigned long vaddr, long npage,
> int prot, 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, i;
> + long ret, i = 1;
> bool rsvd;
>
> if (!current->mm)
> @@ -283,16 +262,11 @@ static long vfio_pin_pages(unsigned long vaddr, long npage,
> return -ENOMEM;
> }
>
> - if (unlikely(disable_hugepages)) {
> - if (!rsvd)
> - vfio_lock_acct(1);
> - return 1;
> - }
> + if (unlikely(disable_hugepages))
> + goto out;
>
> /* Lock all the consecutive pages from pfn_base */
> - for (i = 1, vaddr += PAGE_SIZE; i < npage; i++, vaddr += PAGE_SIZE) {
> - unsigned long pfn = 0;
> -
> + for (vaddr += PAGE_SIZE; i < npage; i++, vaddr += PAGE_SIZE) {
> ret = vaddr_get_pfn(vaddr, prot, &pfn);
> if (ret)
> break;
> @@ -308,12 +282,24 @@ static long vfio_pin_pages(unsigned long vaddr, long npage,
> put_pfn(pfn, prot);
> pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
> __func__, limit << PAGE_SHIFT);
> - break;
> + ret = -ENOMEM;
> + goto unpin_out;
> }
> }
>
> +out:
> if (!rsvd)
> - vfio_lock_acct(i);
> + ret = vfio_lock_acct(i, &lock_cap);
> +
> +unpin_out:
> + if (ret) {
> + if (!rsvd) {
> + for (pfn = *pfn_base ; i ; pfn++, i--)
> + put_pfn(pfn, prot);
> + }
> +
> + return ret;
> + }
>
> return i;
> }
> @@ -328,7 +314,7 @@ static long vfio_unpin_pages(unsigned long pfn, long npage,
> unlocked += put_pfn(pfn++, prot);
>
> if (do_accounting)
> - vfio_lock_acct(-unlocked);
> + vfio_lock_acct(-unlocked, NULL);
>
> return unlocked;
> }
> @@ -390,7 +376,7 @@ static void vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma)
> cond_resched();
> }
>
> - vfio_lock_acct(-unlocked);
> + vfio_lock_acct(-unlocked, NULL);
> }
>
> static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
>
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