On Thu, 6 Apr 2017 18:53:04 +0200 Auger Eric <[email protected]> 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: [email protected] > > Signed-off-by: Alex Williamson <[email protected]> > Looks good to me. > > Reviewed-by: Eric Auger <[email protected]> > 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: > >
