Re: [RFC] vfio iommu type1: improve memory pinning process for raw PFN mapping

From: Jason Cai (Xiang Feng)
Date: Tue Feb 27 2018 - 02:45:13 EST

On 27 Feb 2018, at 3:19 AM, Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

> On Sat, 24 Feb 2018 13:44:07 +0800
> jason <jason.cai@xxxxxxxxxxxxxxxxx> wrote:
>
>> When using vfio to pass through a PCIe device (e.g. a GPU card) that
>> has a huge BAR (e.g. 16GB), a lot of cycles are wasted on memory
>> pinning because PFNs of PCI BAR are not backed by struct page, and
>> the corresponding VMA has flags VM_IO|VM_PFNMAP.
>>
>> With this change, memory pinning process will firstly try to figure
>> out whether the corresponding region is a raw PFN mapping, and if so
>> it can skip unnecessary user memory pinning process.
>>
>> Even though it commes with a little overhead, finding vma and testing
>> flags, on each call, it can significantly improve VM's boot up time
>> when passing through devices via VFIO.
>
> Needs a Sign-off, see Documentation/process/submitting-patches.rst
>
>> ---
>> drivers/vfio/vfio_iommu_type1.c | 22 ++++++++++++++++++++++
>> 1 file changed, 22 insertions(+)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index e30e29ae4819..1a471ece3f9c 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -374,6 +374,24 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>> return ret;
>> }
>>
>> +static int try_io_pfnmap(struct mm_struct *mm, unsigned long vaddr, long npage,
>> + unsigned long *pfn)
>> +{
>> + struct vm_area_struct *vma;
>> + int pinned = 0;
>> +
>> + down_read(&mm->mmap_sem);
>> + vma = find_vma_intersection(mm, vaddr, vaddr + 1);
>> + if (vma && vma->vm_flags & (VM_IO | VM_PFNMAP)) {
>> + *pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
>> + if (is_invalid_reserved_pfn(*pfn))
>> + pinned = min(npage, (long)vma_pages(vma));
>> + }
>> + up_read(&mm->mmap_sem);
>> +
>> + return pinned;
>> +}
>> +
>> /*
>> * Attempt to pin pages. We really don't want to track all the pfns and
>> * the iommu can only map chunks of consecutive pfns anyway, so get the
>> @@ -392,6 +410,10 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>> if (!current->mm)
>> return -ENODEV;
>>
>> + ret = try_io_pfnmap(current->mm, vaddr, npage, pfn_base);
>> + if (ret)
>> + return ret;
>> +
>> ret = vaddr_get_pfn(current->mm, vaddr, dma->prot, pfn_base);
>> if (ret)
>> return ret;
>
> I like the idea, but couldn't we integrated it better? For instance,
> does it really make sense to test for this first, the majority of users
> are going to have more regular mappings than PFNMAP mappings. If we
> were to do the above optimization, doesn't the rsvd bits in the
> remainder of the code become cruft? What if we optimized from the point
> where we test the return of vaddr_get_pfn() for a reserved/invalid page?
> Perhaps something like the below (untested, uncompiled) patch. Also
> curious why the above tests VM_IO|VM_PFNMAP while vaddr_get_pfn() only
> tests VM_PFNMAP, we should at least be consistent, but also correct the
> existing function if it's missing a case. Thanks,
>
> Alex
>

Good points! The patch has been updated according to these points.
And Iâve also tested with it. Please see it at the end. Thanks.

> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index e113b2c43be2..425922393316 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -399,7 +399,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> {
> unsigned long pfn = 0;
> long ret, pinned = 0, lock_acct = 0;
> - bool rsvd;
> dma_addr_t iova = vaddr - dma->vaddr + dma->iova;
>
> /* This code path is only user initiated */
> @@ -410,14 +409,23 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> if (ret)
> return ret;
>
> + if (is_invalid_reserved_pfn(*pfn_base)) {
> + struct vm_area_struct *vma;
> +
> + down_read(&mm->mmap_sem);
> + vma = find_vma_intersection(mm, vaddr, vaddr + 1);
> + pinned = min(npage, (long)vma_pages(vma));
> + up_read(&mm->mmap_sem);
> + return pinned;
> + }
> +
> pinned++;
> - rsvd = is_invalid_reserved_pfn(*pfn_base);
>
> /*
> * 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 (!vfio_find_vpfn(dma, iova)) {
> if (!lock_cap && current->mm->locked_vm + 1 > limit) {
> put_pfn(*pfn_base, dma->prot);
> pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n", __func__,
> @@ -437,13 +445,12 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> if (ret)
> break;
>
> - if (pfn != *pfn_base + pinned ||
> - rsvd != is_invalid_reserved_pfn(pfn)) {
> + if (pfn != *pfn_base + pinned) {
> put_pfn(pfn, dma->prot);
> break;
> }
>
> - if (!rsvd && !vfio_find_vpfn(dma, iova)) {
> + if (!vfio_find_vpfn(dma, iova)) {
> if (!lock_cap &&
> current->mm->locked_vm + lock_acct + 1 > limit) {
> put_pfn(pfn, dma->prot);
> @@ -461,10 +468,8 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>
> unpin_out:
> if (ret) {
> - if (!rsvd) {
> - for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> - put_pfn(pfn, dma->prot);
> - }
> + for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> + put_pfn(pfn, dma->prot);
>
> return ret;
> }


When using vfio to pass through a PCIe device (e.g. a GPU card) that
has a huge BAR (e.g. 16GB), a lot of cycles are wasted on memory
pinning because PFNs of PCI BAR are not backed by struct page, and
the corresponding VMA has flag VM_PFNMAP.

With this change, when pinning a region which is a raw PFN mapping,
it can skip unnecessary user memory pinning process. Thus, it can
significantly improve VM's boot up time when passing through devices
via VFIO.

Signed-off-by: Jason Cai (Xiang Feng) <jason.cai@xxxxxxxxxxxxxxxxx>
---
drivers/vfio/vfio_iommu_type1.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index e30e29ae4819..82ccfa350315 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -385,7 +385,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
{
unsigned long pfn = 0;
long ret, pinned = 0, lock_acct = 0;
- bool rsvd;
dma_addr_t iova = vaddr - dma->vaddr + dma->iova;

/* This code path is only user initiated */
@@ -396,14 +395,22 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
if (ret)
return ret;

+ if (is_invalid_reserved_pfn(*pfn_base)) {
+ struct vm_area_struct *vma;
+ down_read(&current->mm->mmap_sem);
+ vma = find_vma_intersection(current->mm, vaddr, vaddr + 1);
+ pinned = min(npage, (long)vma_pages(vma));
+ up_read(&current->mm->mmap_sem);
+ return pinned;
+ }
+
pinned++;
- rsvd = is_invalid_reserved_pfn(*pfn_base);

/*
* 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 (!vfio_find_vpfn(dma, iova)) {
if (!lock_cap && current->mm->locked_vm + 1 > limit) {
put_pfn(*pfn_base, dma->prot);
pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n", __func__,
@@ -423,13 +430,12 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
if (ret)
break;

- if (pfn != *pfn_base + pinned ||
- rsvd != is_invalid_reserved_pfn(pfn)) {
+ if (pfn != *pfn_base + pinned) {
put_pfn(pfn, dma->prot);
break;
}

- if (!rsvd && !vfio_find_vpfn(dma, iova)) {
+ if (!vfio_find_vpfn(dma, iova)) {
if (!lock_cap &&
current->mm->locked_vm + lock_acct + 1 > limit) {
put_pfn(pfn, dma->prot);
@@ -447,10 +453,8 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,

unpin_out:
if (ret) {
- if (!rsvd) {
- for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
- put_pfn(pfn, dma->prot);
- }
+ for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
+ put_pfn(pfn, dma->prot);

return ret;
}
--
2.13.6