Re: [PATCH v4 1/2] vfio/type1: Remove locked page accounting workqueue

Kirti Wankhede
2017-04-17T19:32:12Z

On 4/18/2017 12:49 AM, Alex Williamson wrote:
> On Tue, 18 Apr 2017 00:35:06 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>
>> On 4/17/2017 8:02 PM, Alex Williamson wrote:
>>> On Mon, 17 Apr 2017 14:47:54 +0800
>>> Peter Xu <peterx@redhat.com> wrote:
>>>
>>>> On Sun, Apr 16, 2017 at 07:42:27PM -0600, Alex Williamson wrote:
>>>>
>>>> [...]
>>>>
>>>>> -static void vfio_lock_acct(struct task_struct *task, long npage)
>>>>> +static int vfio_lock_acct(struct task_struct *task, long npage, bool lock_cap)
>>>>> {
>>>>> - 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 || lock_cap) {
>>>>
>>>> Nit: maybe we can avoid passing in lock_cap in all the callers of
>>>> vfio_lock_acct() and fetch it via has_capability() only if npage < 0?
>>>> IMHO that'll keep the vfio_lock_acct() interface cleaner, and we won't
>>>> need to pass in "false" any time when doing unpins.
>>>
>>> Unfortunately vfio_pin_pages_remote() needs to know about lock_cap
>>> since it tests whether the user is exceeding their locked memory
>>> limit. The other callers could certainly get away with
>>> vfio_lock_acct() testing the capability itself but that would add a
>>> redundant call for the most common user. I'm not a big fan of passing
>>> a lock_cap bool either, but it seemed the best fix for now. The
>>> cleanest alternative I can up with is this (untested):
>>>
>>
>> In my opinion, passing 'bool lock_cap' looks much clean and simple.
>>
>> Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
>
> Well shoot, I was just starting to warm up to the bool*. I like that
> we're not presuming the polarity for the callers we expect to be
> removing pages and I generally just dislike passing fixed bool
> parameters to change the function behavior. I've cleaned it up a bit
> further and was starting to do some testing on this which I'd propose
> for v5. Does it change your opinion?

If passing fixed bool parameter is the concern then I would lean towards
Peter's suggestion. vfio_pin_pages_remote() will check lock capability
outside vfio_lock_acct() and again in vfio_lock_acct(). At other places,
it will be takes care within vfio_lock_acct()

Thanks,
Kirti

>
> commit cd61c5f507d614ac14b75b0a548c8738deff88ea
> Author: Alex Williamson <alex.williamson@redhat.com>
> Date: Thu Apr 13 14:10:15 2017 -0600
>
> vfio/type1: Remove locked page accounting workqueue
>
> 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@vger.kernel.org
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 32d2633092a3..a8a079ba9477 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -246,69 +246,46 @@ 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, bool *lock_cap)
> {
> - 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) {
> + if (lock_cap ? !*lock_cap :
> + !has_capability(task, CAP_IPC_LOCK)) {
> + unsigned long limit;
> +
> + limit = task_rlimit(task,
> + RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +
> + if (mm->locked_vm + npage > limit)
> + ret = -ENOMEM;
> + }
> + }
> +
> + if (!ret)
> + mm->locked_vm += npage;
>
> - 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 +382,7 @@ 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 +419,6 @@ 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 +435,25 @@ 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, &lock_cap);
> +
> +unpin_out:
> + if (ret) {
> + if (!rsvd) {
> + for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> + put_pfn(pfn, dma->prot);
> + }
> +
> + return ret;
> + }
>
> return pinned;
> }
> @@ -488,7 +474,7 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> }
>
> if (do_accounting)
> - vfio_lock_acct(dma->task, locked - unlocked);
> + vfio_lock_acct(dma->task, locked - unlocked, NULL);
>
> return unlocked;
> }
> @@ -522,8 +508,14 @@ 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, &lock_cap);
> + if (ret) {
> + put_pfn(*pfn_base, dma->prot);
> + goto pin_page_exit;
> + }
> + }
> +
> ret = 1;
>
> pin_page_exit:
> @@ -543,7 +535,7 @@ static int vfio_unpin_page_external(struct vfio_dma *dma, dma_addr_t iova,
> unlocked = vfio_iova_put_vfio_pfn(dma, vpfn);
>
> if (do_accounting)
> - vfio_lock_acct(dma->task, -unlocked);
> + vfio_lock_acct(dma->task, -unlocked, NULL);
>
> return unlocked;
> }
> @@ -740,7 +732,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>
> dma->iommu_mapped = false;
> if (do_accounting) {
> - vfio_lock_acct(dma->task, -unlocked);
> + vfio_lock_acct(dma->task, -unlocked, NULL);
> return 0;
> }
> return unlocked;
> @@ -1382,7 +1374,7 @@ static void vfio_iommu_unmap_unpin_reaccount(struct vfio_iommu *iommu)
> if (!is_invalid_reserved_pfn(vpfn->pfn))
> locked++;
> }
> - vfio_lock_acct(dma->task, locked - unlocked);
> + vfio_lock_acct(dma->task, locked - unlocked, NULL);
> }
> }
>
>
> Patch 2/2 would clearly change the &lock_cap in
> vfio_pin_page_external() to a NULL, so only _remote passes a pointer
> there. Thanks,
>
> Alex
>