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Message-ID: <20250519100724.7fd6cc1e.alex.williamson@redhat.com>
Date: Mon, 19 May 2025 10:07:24 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: lizhe.67@...edance.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
muchun.song@...ux.dev, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2] vfio/type1: optimize vfio_pin_pages_remote() for
hugetlbfs folio
On Mon, 19 May 2025 15:04:19 +0800
lizhe.67@...edance.com wrote:
> From: Li Zhe <lizhe.67@...edance.com>
>
> When vfio_pin_pages_remote() is called with a range of addresses that
> includes hugetlbfs 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: # 15635.055 us | vfio_pin_map_dma();
>
> Signed-off-by: Li Zhe <lizhe.67@...edance.com>
> ---
> Changelogs:
>
> v1->v2:
> - Fix some issues in comments and formatting.
> - Consolidate vfio_find_vpfn_range() and vfio_find_vpfn().
> - Move the processing logic for hugetlbfs folio into the while(true) loop
> and use a variable with a default value of 1 to indicate the number of
> consecutive pages.
>
> v1 patch: https://lore.kernel.org/all/20250513035730.96387-1-lizhe.67@bytedance.com/
>
> drivers/vfio/vfio_iommu_type1.c | 70 +++++++++++++++++++++++++++------
> 1 file changed, 58 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 0ac56072af9f..2218ca415366 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -317,17 +317,20 @@ static void vfio_dma_bitmap_free_all(struct vfio_iommu *iommu)
> }
>
> /*
> - * Helper Functions for host iova-pfn list
> + * Find the first vfio_pfn that overlapping the range
> + * [iova, iova + PAGE_SIZE * npage) in rb tree
> */
> -static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
> +static struct vfio_pfn *vfio_find_vpfn_range(struct vfio_dma *dma,
> + dma_addr_t iova, unsigned long npage)
> {
> struct vfio_pfn *vpfn;
> struct rb_node *node = dma->pfn_list.rb_node;
> + dma_addr_t end_iova = iova + PAGE_SIZE * npage;
>
> while (node) {
> vpfn = rb_entry(node, struct vfio_pfn, node);
>
> - if (iova < vpfn->iova)
> + if (end_iova <= vpfn->iova)
> node = node->rb_left;
> else if (iova > vpfn->iova)
> node = node->rb_right;
> @@ -337,6 +340,14 @@ static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
> return NULL;
> }
>
> +/*
> + * Helper Functions for host iova-pfn list
> + */
This comment should still precede the renamed function above, it's in
reference to this section of code related to searching, inserting, and
removing entries from the pfn list.
> +static inline struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
> +{
> + return vfio_find_vpfn_range(dma, iova, 1);
> +}
> +
> static void vfio_link_pfn(struct vfio_dma *dma,
> struct vfio_pfn *new)
> {
> @@ -681,32 +692,67 @@ 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) {
> + int page_step = 1;
> + long lock_acct_step = 1;
> + struct folio *folio = page_folio(batch->pages[batch->offset]);
> + bool found_vpfn;
> +
> if (pfn != *pfn_base + pinned ||
> rsvd != is_invalid_reserved_pfn(pfn))
> goto out;
>
> + /* Handle hugetlbfs page */
> + if (folio_test_hugetlb(folio)) {
Why do we care to specifically test for hugetlb vs
folio_large_nr_pages(), at which point we can just use folio_nr_pages()
directly here.
> + unsigned long start_pfn = PHYS_PFN(vaddr);
Using this macro on a vaddr looks wrong.
> +
> + /*
> + * Note: The current page_step does not achieve the optimal
> + * performance in scenarios where folio_nr_pages() exceeds
> + * batch->capacity. It is anticipated that future enhancements
> + * will address this limitation.
> + */
> + page_step = min(batch->size,
> + ALIGN(start_pfn + 1, folio_nr_pages(folio)) - start_pfn);
Why do we assume start_pfn is the beginning of the folio?
> + found_vpfn = !!vfio_find_vpfn_range(dma, iova, page_step);
> + if (rsvd || !found_vpfn) {
> + lock_acct_step = page_step;
> + } else {
> + dma_addr_t tmp_iova = iova;
> + int i;
> +
> + lock_acct_step = 0;
> + for (i = 0; i < page_step; ++i, tmp_iova += PAGE_SIZE)
> + if (!vfio_find_vpfn(dma, tmp_iova))
> + lock_acct_step++;
> + if (lock_acct_step)
> + found_vpfn = false;
Why are we making this so complicated versus falling back to iterating
at page per page?
> + }
> + } else {
> + found_vpfn = vfio_find_vpfn(dma, iova);
> + }
> +
> /*
> * 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 (!rsvd && !found_vpfn) {
> if (!dma->lock_cap &&
> - mm->locked_vm + lock_acct + 1 > limit) {
> + mm->locked_vm + lock_acct + lock_acct_step > limit) {
> pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
> __func__, limit << PAGE_SHIFT);
> ret = -ENOMEM;
> goto unpin_out;
> }
> - lock_acct++;
> + lock_acct += lock_acct_step;
> }
>
> - pinned++;
> - npage--;
> - vaddr += PAGE_SIZE;
> - iova += PAGE_SIZE;
> - batch->offset++;
> - batch->size--;
> + pinned += page_step;
> + npage -= page_step;
> + vaddr += PAGE_SIZE * page_step;
> + iova += PAGE_SIZE * page_step;
> + batch->offset += page_step;
> + batch->size -= page_step;
>
> if (!batch->size)
> break;
Why is something like below (untested) not sufficient?
NB. (vaddr - folio_address()) still needs some scrutiny to determine if
it's valid.
@@ -681,32 +692,40 @@ 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) {
+ struct folio *folio = page_folio(batch->pages[batch->offset]);
+ long nr_pages;
+
if (pfn != *pfn_base + pinned ||
rsvd != is_invalid_reserved_pfn(pfn))
goto out;
+ nr_pages = min(batch->size, folio_nr_pages(folio) -
+ (vaddr - folio_address(folio)) >> PAGE_SHIFT);
+ if (nr_pages > 1 && vfio_find_vpfn_range(dma, iova, nr_pages))
+ nr_pages = 1;
+
/*
* 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 (!rsvd && (nr_pages > 1 || !vfio_find_vpfn(dma, iova))) {
if (!dma->lock_cap &&
- mm->locked_vm + lock_acct + 1 > limit) {
+ mm->locked_vm + lock_acct + nr_pages > limit) {
pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
__func__, limit << PAGE_SHIFT);
ret = -ENOMEM;
goto unpin_out;
}
- lock_acct++;
+ lock_acct += nr_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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