[PATCH v4 3/3] vfio/type1: optimize vfio_unpin_pages_remote() for large folio

[lizhe.67@bytedance.com]

From: Li Zhe <lizhe.67@bytedance.com>

When vfio_unpin_pages_remote() is called with a range of addresses that
includes large folios, the function currently performs individual
put_pfn() 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 put_pfn() operations.

The performance test results, based on v6.15, for completing the 16G VFIO
IOMMU DMA unmapping, obtained through unit test[1] with slight
modifications[2], are as follows.

Base(v6.15):
./vfio-pci-mem-dma-map 0000:03:00.0 16
------- AVERAGE (MADV_HUGEPAGE) --------
VFIO MAP DMA in 0.047 s (338.6 GB/s)
VFIO UNMAP DMA in 0.138 s (116.2 GB/s)
------- AVERAGE (MAP_POPULATE) --------
VFIO MAP DMA in 0.280 s (57.2 GB/s)
VFIO UNMAP DMA in 0.312 s (51.3 GB/s)
------- AVERAGE (HUGETLBFS) --------
VFIO MAP DMA in 0.052 s (308.3 GB/s)
VFIO UNMAP DMA in 0.139 s (115.1 GB/s)

Map[3] + This patchset:
------- AVERAGE (MADV_HUGEPAGE) --------
VFIO MAP DMA in 0.028 s (563.9 GB/s)
VFIO UNMAP DMA in 0.049 s (325.1 GB/s)
------- AVERAGE (MAP_POPULATE) --------
VFIO MAP DMA in 0.294 s (54.4 GB/s)
VFIO UNMAP DMA in 0.296 s (54.1 GB/s)
------- AVERAGE (HUGETLBFS) --------
VFIO MAP DMA in 0.033 s (485.1 GB/s)
VFIO UNMAP DMA in 0.049 s (324.4 GB/s)

For large folio, we achieve an approximate 64% performance improvement
in the VFIO UNMAP DMA item. For small folios, the performance test
results appear to show no significant changes.

[1]: https://github.com/awilliam/tests/blob/vfio-pci-mem-dma-map/vfio-pci-mem-dma-map.c
[2]: https://lore.kernel.org/all/20250610031013.98556-1-lizhe.67@bytedance.com/
[3]: https://lore.kernel.org/all/20250529064947.38433-1-lizhe.67@bytedance.com/

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 drivers/vfio/vfio_iommu_type1.c | 35 +++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index e952bf8bdfab..159ba80082a8 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -806,11 +806,38 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
 				    bool do_accounting)
 {
 	long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
-	long i;
 
-	for (i = 0; i < npage; i++)
-		if (put_pfn(pfn++, dma->prot))
-			unlocked++;
+	while (npage) {
+		long nr_pages = 1;
+
+		if (!is_invalid_reserved_pfn(pfn)) {
+			struct page *page = pfn_to_page(pfn);
+			struct folio *folio = page_folio(page);
+			long folio_pages_num = folio_nr_pages(folio);
+
+			/*
+			 * For a folio, it represents a physically
+			 * contiguous set of bytes, and all of its pages
+			 * share the same invalid/reserved state.
+			 *
+			 * Here, our PFNs are contiguous. Therefore, if we
+			 * detect that the current PFN belongs to a large
+			 * folio, we can batch the operations for the next
+			 * nr_pages PFNs.
+			 */
+			if (folio_pages_num > 1)
+				nr_pages = min_t(long, npage,
+					folio_pages_num -
+					folio_page_idx(folio, page));
+
+			unpin_user_folio_dirty_locked(folio, nr_pages,
+					dma->prot & IOMMU_WRITE);
+			unlocked += nr_pages;
+		}
+
+		pfn += nr_pages;
+		npage -= nr_pages;
+	}
 
 	if (do_accounting)
 		vfio_lock_acct(dma, locked - unlocked, true);
-- 
2.20.1