On 21.05.25 00:21, Alex Williamson wrote:
On Tue, 20 May 2025 19:38:45 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:
On 20.05.25 09:00, lizhe.67@xxxxxxxxxxxxx wrote:
From: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
Subject: "huge folio" -> "large folios"
When vfio_pin_pages_remote() is called with a range of addresses that
includes huge folios, the function currently performs individual
Similar, we call it a "large" f
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@xxxxxxxxxxxxx>
Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
---
Changelogs:
v2->v3:
- Code simplification.
- Fix some issues in comments.
v1->v2:
- Fix some issues in comments and formatting.
- Consolidate vfio_find_vpfn_range() and vfio_find_vpfn().
- Move the processing logic for huge folio into the while(true) loop
and use a variable with a default value of 1 to indicate the number
of consecutive pages.
v2 patch: https://lore.kernel.org/all/20250519070419.25827-1-lizhe.67@xxxxxxxxxxxxx/
v1 patch: https://lore.kernel.org/all/20250513035730.96387-1-lizhe.67@xxxxxxxxxxxxx/
drivers/vfio/vfio_iommu_type1.c | 48 +++++++++++++++++++++++++--------
1 file changed, 37 insertions(+), 11 deletions(-)
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 0ac56072af9f..48f06ce0e290 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -319,15 +319,22 @@ 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, iova + PAGE_SIZE * npage) in rb tree.
+ */
+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 +344,11 @@ 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, 1);
+}
+
static void vfio_link_pfn(struct vfio_dma *dma,
struct vfio_pfn *new)
{
@@ -681,32 +693,46 @@ 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;
+ /*
+ * Note: The current nr_pages 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.
+ */
+ nr_pages = min((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;
You seem to assume that the batch really contains the consecutive pages
of that folio.
I don't think we are. We're iterating through our batch of pages from
GUP to find consecutive pfns. We use the page to get the pfn, the
folio, and immediately above, the offset into the folio. batch->size is
the remaining length of the page array from GUP and batch->offset is our
current index into that array.
Let me try again using an example below ....
This is not the case if we obtained the pages through GUP and we have
(a) A MAP_PRIVATE mapping
(b) We span multiple different VMAs
Are we sure we can rule out (a) and (b)?
A more future-proof approach would be at least looking whether the
pages/pfns are actually consecutive.
The unmodified (pfn != *pfn_base + pinned) test is where we verify we
have the next consecutive pfn. Maybe I'm not catching the dependency
you're seeing on consecutive pages, I think there isn't one unless
we're somehow misusing folio_page_idx() to get the offset into the
folio.
Assume our page tables look like this (case (a), a partially mapped
large pagecache folio mixed with COW'ed anonymous folios):
+ page[0] of folio 0
| + COWed anonymous folio (folio 1)
| | + page[4] of folio 0
| | |
v v v
F0P0 F0P1 F0P2 F1P0 F0P4 P0P5 F0P6 F0P7
If we GUP that range, we get exactly these pages, except that the PFNs
are not consecutive, because F0P3 was replaced by another page. The
large folio is partially mapped.
Maybe I misunderstand that code, but wouldn't we just "jump" over F1P0
because we assume the batch would contain F1P0, where it really contains
F0P4?
--
Cheers,
David / dhildenb
