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Message-ID: <20250522145207.01734386.alex.williamson@redhat.com>
Date: Thu, 22 May 2025 14:52:07 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: lizhe.67@...edance.com
Cc: david@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
muchun.song@...ux.dev, peterx@...hat.com
Subject: Re: [PATCH v4] vfio/type1: optimize vfio_pin_pages_remote() for
large folio
On Thu, 22 May 2025 16:25:24 +0800
lizhe.67@...edance.com wrote:
> On Thu, 22 May 2025 09:22:50 +0200, david@...hat.com wrote:
>
> >On 22.05.25 05:49, lizhe.67@...edance.com wrote:
> >> On Wed, 21 May 2025 13:17:11 -0600, alex.williamson@...hat.com wrote:
> >>
> >>>> From: Li Zhe <lizhe.67@...edance.com>
> >>>>
> >>>> When vfio_pin_pages_remote() is called with a range of addresses that
> >>>> includes large folios, the function currently performs individual
> >>>> statistics counting operations for each page. This can lead to significant
> >>>> performance overheads, especially when dealing with large ranges of pages.
> >>>>
> >>>> This patch optimize this process by batching the statistics counting
> >>>> operations.
> >>>>
> >>>> The performance test results for completing the 8G VFIO IOMMU DMA mapping,
> >>>> obtained through trace-cmd, are as follows. In this case, the 8G virtual
> >>>> address space has been mapped to physical memory using hugetlbfs with
> >>>> pagesize=2M.
> >>>>
> >>>> Before this patch:
> >>>> funcgraph_entry: # 33813.703 us | vfio_pin_map_dma();
> >>>>
> >>>> After this patch:
> >>>> funcgraph_entry: # 16071.378 us | vfio_pin_map_dma();
> >>>>
> >>>> Signed-off-by: Li Zhe <lizhe.67@...edance.com>
> >>>> Co-developed-by: Alex Williamson <alex.williamson@...hat.com>
> >>>> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> >>>> ---
> >>>
> >>> Given the discussion on v3, this is currently a Nak. Follow-up in that
> >>> thread if there are further ideas how to salvage this. Thanks,
> >>
> >> How about considering the solution David mentioned to check whether the
> >> pages or PFNs are actually consecutive?
> >>
> >> I have conducted a preliminary attempt, and the performance testing
> >> revealed that the time consumption is approximately 18,000 microseconds.
> >> Compared to the previous 33,000 microseconds, this also represents a
> >> significant improvement.
> >>
> >> The modification is quite straightforward. The code below reflects the
> >> changes I have made based on this patch.
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index bd46ed9361fe..1cc1f76d4020 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -627,6 +627,19 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
> >> return ret;
> >> }
> >>
> >> +static inline long continuous_page_num(struct vfio_batch *batch, long npage)
> >> +{
> >> + long i;
> >> + unsigned long next_pfn = page_to_pfn(batch->pages[batch->offset]) + 1;
> >> +
> >> + for (i = 1; i < npage; ++i) {
> >> + if (page_to_pfn(batch->pages[batch->offset + i]) != next_pfn)
> >> + break;
> >> + next_pfn++;
> >> + }
> >> + return i;
> >> +}
> >
> >
> >What might be faster is obtaining the folio, and then calculating the
> >next expected page pointer, comparing whether the page pointers match.
> >
> >Essentially, using folio_page() to calculate the expected next page.
> >
> >nth_page() is a simple pointer arithmetic with CONFIG_SPARSEMEM_VMEMMAP,
> >so that might be rather fast.
> >
> >
> >So we'd obtain
> >
> >start_idx = folio_idx(folio, batch->pages[batch->offset]);
>
> Do you mean using folio_page_idx()?
>
> >and then check for
> >
> >batch->pages[batch->offset + i] == folio_page(folio, start_idx + i)
>
> Thank you for your reminder. This is indeed a better solution.
> The updated code might look like this:
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index bd46ed9361fe..f9a11b1d8433 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -627,6 +627,20 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
> return ret;
> }
>
> +static inline long continuous_pages_num(struct folio *folio,
> + struct vfio_batch *batch, long npage)
Note this becomes long enough that we should just let the compiler
decide whether to inline or not.
> +{
> + long i;
> + unsigned long start_idx =
> + folio_page_idx(folio, batch->pages[batch->offset]);
> +
> + for (i = 1; i < npage; ++i)
> + if (batch->pages[batch->offset + i] !=
> + folio_page(folio, start_idx + i))
> + break;
> + return i;
> +}
> +
> /*
> * 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
> @@ -708,8 +722,12 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> */
> nr_pages = min_t(long, batch->size, folio_nr_pages(folio) -
> folio_page_idx(folio, batch->pages[batch->offset]));
> - if (nr_pages > 1 && vfio_find_vpfn_range(dma, iova, nr_pages))
> - nr_pages = 1;
> + if (nr_pages > 1) {
> + if (vfio_find_vpfn_range(dma, iova, nr_pages))
> + nr_pages = 1;
> + else
> + nr_pages = continuous_pages_num(folio, batch, nr_pages);
> + }
I think we can refactor this a bit better and maybe if we're going to
the trouble of comparing pages we can be a bit more resilient to pages
already accounted as vpfns. I took a shot at it, compile tested only,
is there still a worthwhile gain?
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 0ac56072af9f..e8bba32148f7 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -319,7 +319,13 @@ static void vfio_dma_bitmap_free_all(struct vfio_iommu *iommu)
/*
* Helper Functions for host iova-pfn list
*/
-static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
+
+/*
+ * Find the first vfio_pfn that overlapping the range
+ * [iova_start, iova_end) in rb tree.
+ */
+static struct vfio_pfn *vfio_find_vpfn_range(struct vfio_dma *dma,
+ dma_addr_t iova_start, dma_addr_t iova_end)
{
struct vfio_pfn *vpfn;
struct rb_node *node = dma->pfn_list.rb_node;
@@ -327,9 +333,9 @@ static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
while (node) {
vpfn = rb_entry(node, struct vfio_pfn, node);
- if (iova < vpfn->iova)
+ if (iova_end <= vpfn->iova)
node = node->rb_left;
- else if (iova > vpfn->iova)
+ else if (iova_start > vpfn->iova)
node = node->rb_right;
else
return vpfn;
@@ -337,6 +343,11 @@ static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
return NULL;
}
+static inline struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
+{
+ return vfio_find_vpfn_range(dma, iova, iova + PAGE_SIZE);
+}
+
static void vfio_link_pfn(struct vfio_dma *dma,
struct vfio_pfn *new)
{
@@ -615,6 +626,43 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
return ret;
}
+static long contig_pages(struct vfio_dma *dma,
+ struct vfio_batch *batch, dma_addr_t iova)
+{
+ struct page *page = batch->pages[batch->offset];
+ struct folio *folio = page_folio(page);
+ long idx = folio_page_idx(folio, page);
+ long max = min_t(long, batch->size, folio_nr_pages(folio) - idx);
+ long nr_pages;
+
+ for (nr_pages = 1; nr_pages < max; nr_pages++) {
+ if (batch->pages[batch->offset + nr_pages] !=
+ folio_page(folio, idx + nr_pages))
+ break;
+ }
+
+ return nr_pages;
+}
+
+static long vpfn_pages(struct vfio_dma *dma,
+ dma_addr_t iova_start, long nr_pages)
+{
+ dma_addr_t iova_end = iova_start + (nr_pages << PAGE_SHIFT);
+ struct vfio_pfn *vpfn;
+ long count = 0;
+
+ do {
+ vpfn = vfio_find_vpfn_range(dma, iova_start, iova_end);
+ if (likely(!vpfn))
+ break;
+
+ count++;
+ iova_start = vpfn->iova + PAGE_SIZE;
+ } while (iova_start < iova_end);
+
+ return count;
+}
+
/*
* 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
@@ -681,32 +729,40 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
* and rsvd here, and therefore continues to use the batch.
*/
while (true) {
+ long nr_pages, acct_pages = 0;
+
if (pfn != *pfn_base + pinned ||
rsvd != is_invalid_reserved_pfn(pfn))
goto out;
+ nr_pages = contig_pages(dma, batch, iova);
+ if (!rsvd) {
+ acct_pages = nr_pages;
+ acct_pages -= vpfn_pages(dma, iova, nr_pages);
+ }
+
/*
* 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 (acct_pages) {
if (!dma->lock_cap &&
- mm->locked_vm + lock_acct + 1 > limit) {
+ mm->locked_vm + lock_acct + acct_pages > limit) {
pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
__func__, limit << PAGE_SHIFT);
ret = -ENOMEM;
goto unpin_out;
}
- lock_acct++;
+ lock_acct += acct_pages;
}
- pinned++;
- npage--;
- vaddr += PAGE_SIZE;
- iova += PAGE_SIZE;
- batch->offset++;
- batch->size--;
+ pinned += nr_pages;
+ npage -= nr_pages;
+ vaddr += PAGE_SIZE * nr_pages;
+ iova += PAGE_SIZE * nr_pages;
+ batch->offset += nr_pages;
+ batch->size -= nr_pages;
if (!batch->size)
break;
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