linux-kernel - [PATCH] Revert "vfio/type1: Unpin zero pages"

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Date: Thu, 29 Feb 2024 15:35:40 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: alex.williamson@...hat.com
Cc: david@...hat.com,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] Revert "vfio/type1: Unpin zero pages"

This reverts commit 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4.

This was a heinous workaround and it turns out it's been fixed in mm
twice since it was introduced.  Most recently, commit c8070b787519
("mm: Don't pin ZERO_PAGE in pin_user_pages()") would have prevented
running up the zeropage refcount, but even before that commit
84209e87c696 ("mm/gup: reliable R/O long-term pinning in COW mappings")
avoids the vfio use case from pinning the zeropage at all, instead
replacing it with exclusive anonymous pages.

Remove this now useless overhead.

Suggested-by: David Hildenbrand <david@...hat.com>
Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
---
 drivers/vfio/vfio_iommu_type1.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index b2854d7939ce..b5c15fe8f9fc 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -567,18 +567,6 @@ static int vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
 	ret = pin_user_pages_remote(mm, vaddr, npages, flags | FOLL_LONGTERM,
 				    pages, NULL);
 	if (ret > 0) {
-		int i;
-
-		/*
-		 * The zero page is always resident, we don't need to pin it
-		 * and it falls into our invalid/reserved test so we don't
-		 * unpin in put_pfn().  Unpin all zero pages in the batch here.
-		 */
-		for (i = 0 ; i < ret; i++) {
-			if (unlikely(is_zero_pfn(page_to_pfn(pages[i]))))
-				unpin_user_page(pages[i]);
-		}
-
 		*pfn = page_to_pfn(pages[0]);
 		goto done;
 	}
-- 
2.43.2