Re: [PATCH v4 2/3] gup: introduce unpin_user_folio_dirty_locked()

From: David Hildenbrand
Date: Tue Jun 17 2025 - 05:28:47 EST

On 17.06.25 11:21, lizhe.67@xxxxxxxxxxxxx wrote:
On Tue, 17 Jun 2025 09:43:56 +0200, david@xxxxxxxxxx wrote:
On 17.06.25 06:18, lizhe.67@xxxxxxxxxxxxx wrote:
From: Li Zhe <lizhe.67@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx/
[3]: https://lore.kernel.org/all/20250529064947.38433-1-lizhe.67@xxxxxxxxxxxxx/

Signed-off-by: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
---
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));
+

(I know I can be a pain :) )

No, not at all! I really appreciate you taking the time to review my
patch.

But the long comment indicates that this is confusing.


That is essentially the logic in gup_folio_range_next().

What about factoring that out into a helper like

/*
* TODO, returned number includes the provided current page.
*/
unsigned long folio_remaining_pages(struct folio *folio,
struct pages *pages, unsigned long max_pages)
{
if (!folio_test_large(folio))
return 1;
return min_t(unsigned long, max_pages,
folio_nr_pages(folio) - folio_page_idx(folio, page));
}


Then here you would do

if (!is_invalid_reserved_pfn(pfn)) {
struct page *page = pfn_to_page(pfn);
struct folio *folio = page_folio(page);

/* We can batch-process pages belonging to the same folio. */
nr_pages = folio_remaining_pages(folio, page, npage);

unpin_user_folio_dirty_locked(folio, nr_pages,
dma->prot & IOMMU_WRITE);
unlocked += nr_pages;
}

Yes, this indeed makes the code much more comprehensible. Do you think
the implementation of the patch as follows look viable to you? I have
added some brief comments on top of your work to explain why we can
batch-process pages belonging to the same folio. This was suggested by
Alex[1].

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index e952bf8bdfab..d7653f4c10d5 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -801,16 +801,43 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
return pinned;
}
+/* Returned number includes the provided current page. */
+static inline unsigned long folio_remaining_pages(struct folio *folio,
+ struct page *page, unsigned long max_pages)
+{
+ if (!folio_test_large(folio))
+ return 1;
+ return min_t(unsigned long, max_pages,
+ folio_nr_pages(folio) - folio_page_idx(folio, page));
+}

Note that I think that should go somewhere into mm.h, and also get used by GUP. So factoring it out from GUP and then using it here.

--
Cheers,

David / dhildenb