Re: [PATCH] vfio/type1: Restore mapping performance with mdev support
From: Kirti Wankhede
Date: Thu Dec 15 2016 - 01:36:14 EST
On 12/14/2016 2:28 AM, Alex Williamson wrote:
> As part of the mdev support, type1 now gets a task reference per
> vfio_dma and uses that to get an mm reference for the task while
> working on accounting. That's the correct thing to do for paths
> where we can't rely on using current, but there are still hot paths
> where we can optimize because we know we're invoked by the user.
>
> Specifically, vfio_pin_pages_remote() is only called when the user
> does DMA mapping (vfio_dma_do_map) or if an IOMMU group is added to
> a container with existing mappings (vfio_iommu_replay). We can
> therefore use current->mm as well as rlimit() and capable() directly
> rather than going through the high overhead path via the stored
> task_struct. We also know that vfio_dma_do_unmap() is only called
> via user ioctl, so we can also tune that path to be more lightweight.
>
> In a synthetic guest mapping test emulating a 1TB VM backed by a
> single 4GB range remapped multiple times across the address space,
> the mdev changes to the type1 backend introduced a roughly 25% hit
> in runtime of this test. These changes restore it to nearly the
> previous performance for the interfaces exercised here,
> VFIO_IOMMU_MAP_DMA and release on close.
>
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
> drivers/vfio/vfio_iommu_type1.c | 145 +++++++++++++++++++++------------------
> 1 file changed, 79 insertions(+), 66 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 9815e45..8dfeafb 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -103,6 +103,10 @@ struct vfio_pfn {
> #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu) \
> (!list_empty(&iommu->domain_list))
>
> +/* Make function bool options readable */
> +#define IS_CURRENT (true)
> +#define DO_ACCOUNTING (true)
> +
> static int put_pfn(unsigned long pfn, int prot);
>
> /*
> @@ -264,7 +268,8 @@ static void vfio_lock_acct_bg(struct work_struct *work)
> kfree(vwork);
> }
>
> -static void vfio_lock_acct(struct task_struct *task, long npage)
> +static void vfio_lock_acct(struct task_struct *task,
> + long npage, bool is_current)
> {
> struct vwork *vwork;
> struct mm_struct *mm;
> @@ -272,24 +277,31 @@ static void vfio_lock_acct(struct task_struct *task, long npage)
> if (!npage)
> return;
>
> - mm = get_task_mm(task);
> + mm = is_current ? task->mm : get_task_mm(task);
> if (!mm)
> - return; /* process exited or nothing to do */
> + return; /* process exited */
>
> if (down_write_trylock(&mm->mmap_sem)) {
> mm->locked_vm += npage;
> up_write(&mm->mmap_sem);
> - mmput(mm);
> + if (!is_current)
> + mmput(mm);
> return;
> }
>
> + if (is_current) {
> + mm = get_task_mm(task);
> + if (!mm)
> + 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) {
> + if (WARN_ON(!vwork)) {
> mmput(mm);
> return;
> }
> @@ -345,13 +357,13 @@ static int put_pfn(unsigned long pfn, int prot)
> }
>
> static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> - int prot, unsigned long *pfn)
> + int prot, unsigned long *pfn, bool is_current)
> {
> struct page *page[1];
> struct vm_area_struct *vma;
> int ret;
>
> - if (mm == current->mm) {
> + if (is_current) {
With this change, if vfio_pin_page_external() gets called from QEMU
process context, for example in response to some BAR0 register access,
it will still fallback to slow path, get_user_pages_remote(). We don't
have to change this function. This path already takes care of taking
best possible path.
That also makes me think, vfio_pin_page_external() uses task structure
to get mlock limit and capability. Expectation is mdev vendor driver
shouldn't pin all system memory, but if any mdev driver does that, then
that driver might see such performance impact. Should we optimize this
path if (dma->task == current)?
Thanks,
Kirti
> ret = get_user_pages_fast(vaddr, 1, !!(prot & IOMMU_WRITE),
> page);
> } else {
> @@ -393,96 +405,92 @@ 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;
> - bool lock_cap = ns_capable(task_active_pid_ns(dma->task)->user_ns,
> - CAP_IPC_LOCK);
> - struct mm_struct *mm;
> - long ret, i = 0, lock_acct = 0;
> + unsigned long limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> + bool lock_cap = capable(CAP_IPC_LOCK);
> + long ret, pinned = 0, lock_acct = 0;
> bool rsvd;
> dma_addr_t iova = vaddr - dma->vaddr + dma->iova;
>
> - mm = get_task_mm(dma->task);
> - if (!mm)
> + /* This code path is only user initiated */
> + if (!current->mm)
> return -ENODEV;
>
> - ret = vaddr_get_pfn(mm, vaddr, dma->prot, pfn_base);
> + ret = vaddr_get_pfn(current->mm, vaddr,
> + dma->prot, pfn_base, IS_CURRENT);
> if (ret)
> - goto pin_pg_remote_exit;
> + return ret;
>
> + pinned++;
> rsvd = is_invalid_reserved_pfn(*pfn_base);
> - limit = task_rlimit(dma->task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>
> /*
> * Reserved pages aren't counted against the user, externally pinned
> * pages are already counted against the user.
> */
> if (!rsvd && !vfio_find_vpfn(dma, iova)) {
> - if (!lock_cap && mm->locked_vm + 1 > limit) {
> + if (!lock_cap && current->mm->locked_vm + 1 > limit) {
> put_pfn(*pfn_base, dma->prot);
> pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n", __func__,
> limit << PAGE_SHIFT);
> - ret = -ENOMEM;
> - goto pin_pg_remote_exit;
> + return -ENOMEM;
> }
> lock_acct++;
> }
>
> - i++;
> - if (likely(!disable_hugepages)) {
> - /* Lock all the consecutive pages from pfn_base */
> - for (vaddr += PAGE_SIZE, iova += PAGE_SIZE; i < npage;
> - i++, vaddr += PAGE_SIZE, iova += PAGE_SIZE) {
> - unsigned long pfn = 0;
> + if (unlikely(disable_hugepages))
> + goto out;
>
> - ret = vaddr_get_pfn(mm, vaddr, dma->prot, &pfn);
> - if (ret)
> - break;
> + /* 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;
>
> - if (pfn != *pfn_base + i ||
> - rsvd != is_invalid_reserved_pfn(pfn)) {
> + ret = vaddr_get_pfn(current->mm, vaddr,
> + dma->prot, &pfn, IS_CURRENT);
> + if (ret)
> + break;
> +
> + if (pfn != *pfn_base + pinned ||
> + rsvd != is_invalid_reserved_pfn(pfn)) {
> + put_pfn(pfn, dma->prot);
> + break;
> + }
> +
> + if (!rsvd && !vfio_find_vpfn(dma, iova)) {
> + if (!lock_cap &&
> + current->mm->locked_vm + lock_acct + 1 > limit) {
> put_pfn(pfn, dma->prot);
> + pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
> + __func__, limit << PAGE_SHIFT);
> break;
> }
> -
> - if (!rsvd && !vfio_find_vpfn(dma, iova)) {
> - if (!lock_cap &&
> - mm->locked_vm + lock_acct + 1 > limit) {
> - put_pfn(pfn, dma->prot);
> - pr_warn("%s: RLIMIT_MEMLOCK (%ld) "
> - "exceeded\n", __func__,
> - limit << PAGE_SHIFT);
> - break;
> - }
> - lock_acct++;
> - }
> + lock_acct++;
> }
> }
>
> - vfio_lock_acct(dma->task, lock_acct);
> - ret = i;
> +out:
> + vfio_lock_acct(current, lock_acct, IS_CURRENT);
>
> -pin_pg_remote_exit:
> - mmput(mm);
> - return ret;
> + return pinned;
> }
>
> static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> unsigned long pfn, long npage,
> - bool do_accounting)
> + bool do_accounting, bool is_current)
> {
> long unlocked = 0, locked = 0;
> long i;
>
> - for (i = 0; i < npage; i++) {
> + for (i = 0; i < npage; i++, iova += PAGE_SIZE) {
> if (put_pfn(pfn++, dma->prot)) {
> unlocked++;
> - if (vfio_find_vpfn(dma, iova + (i << PAGE_SHIFT)))
> + if (vfio_find_vpfn(dma, iova))
> locked++;
> }
> }
>
> if (do_accounting)
> - vfio_lock_acct(dma->task, locked - unlocked);
> + vfio_lock_acct(dma->task, locked - unlocked, is_current);
>
> return unlocked;
> }
> @@ -501,7 +509,7 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> if (!mm)
> return -ENODEV;
>
> - ret = vaddr_get_pfn(mm, vaddr, dma->prot, pfn_base);
> + ret = vaddr_get_pfn(mm, vaddr, dma->prot, pfn_base, !IS_CURRENT);
> if (ret)
> goto pin_page_exit;
>
> @@ -518,7 +526,7 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> }
>
> if (!rsvd && do_accounting)
> - vfio_lock_acct(dma->task, 1);
> + vfio_lock_acct(dma->task, 1, !IS_CURRENT);
> ret = 1;
>
> pin_page_exit:
> @@ -538,7 +546,7 @@ 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, !IS_CURRENT);
>
> return unlocked;
> }
> @@ -671,7 +679,7 @@ static int vfio_iommu_type1_unpin_pages(void *iommu_data,
> }
>
> static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> - bool do_accounting)
> + bool do_accounting, bool is_current)
> {
> dma_addr_t iova = dma->iova, end = dma->iova + dma->size;
> struct vfio_domain *domain, *d;
> @@ -727,7 +735,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> unlocked += vfio_unpin_pages_remote(dma, iova,
> phys >> PAGE_SHIFT,
> unmapped >> PAGE_SHIFT,
> - false);
> + !DO_ACCOUNTING, is_current);
> iova += unmapped;
>
> cond_resched();
> @@ -735,15 +743,16 @@ 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, is_current);
> return 0;
> }
> return unlocked;
> }
>
> -static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
> +static void vfio_remove_dma(struct vfio_iommu *iommu,
> + struct vfio_dma *dma, bool is_current)
> {
> - vfio_unmap_unpin(iommu, dma, true);
> + vfio_unmap_unpin(iommu, dma, DO_ACCOUNTING, is_current);
> vfio_unlink_dma(iommu, dma);
> put_task_struct(dma->task);
> kfree(dma);
> @@ -874,7 +883,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> goto again;
> }
> unmapped += dma->size;
> - vfio_remove_dma(iommu, dma);
> + vfio_remove_dma(iommu, dma, IS_CURRENT);
> }
>
> unlock:
> @@ -964,7 +973,8 @@ static int vfio_pin_map_dma(struct vfio_iommu *iommu, struct vfio_dma *dma,
> dma->prot);
> if (ret) {
> vfio_unpin_pages_remote(dma, iova + dma->size, pfn,
> - npage, true);
> + npage, DO_ACCOUNTING,
> + IS_CURRENT);
> break;
> }
>
> @@ -975,7 +985,7 @@ static int vfio_pin_map_dma(struct vfio_iommu *iommu, struct vfio_dma *dma,
> dma->iommu_mapped = true;
>
> if (ret)
> - vfio_remove_dma(iommu, dma);
> + vfio_remove_dma(iommu, dma, IS_CURRENT);
>
> return ret;
> }
> @@ -1322,7 +1332,9 @@ static void vfio_iommu_unmap_unpin_all(struct vfio_iommu *iommu)
> struct rb_node *node;
>
> while ((node = rb_first(&iommu->dma_list)))
> - vfio_remove_dma(iommu, rb_entry(node, struct vfio_dma, node));
> + vfio_remove_dma(iommu,
> + rb_entry(node, struct vfio_dma, node),
> + !IS_CURRENT);
> }
>
> static void vfio_iommu_unmap_unpin_reaccount(struct vfio_iommu *iommu)
> @@ -1335,7 +1347,8 @@ static void vfio_iommu_unmap_unpin_reaccount(struct vfio_iommu *iommu)
> long locked = 0, unlocked = 0;
>
> dma = rb_entry(n, struct vfio_dma, node);
> - unlocked += vfio_unmap_unpin(iommu, dma, false);
> + unlocked += vfio_unmap_unpin(iommu, dma,
> + !DO_ACCOUNTING, !IS_CURRENT);
> p = rb_first(&dma->pfn_list);
> for (; p; p = rb_next(p)) {
> struct vfio_pfn *vpfn = rb_entry(p, struct vfio_pfn,
> @@ -1344,7 +1357,7 @@ 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, !IS_CURRENT);
> }
> }
>
>