[PATCH 14/19] vfio, mm: pin_longterm_pages (FOLL_PIN) and put_user_page() conversion

[John Hubbard <jhubbard@nvidia.com>]

This also fixes one or two likely bugs.

1. Change vfio from get_user_pages(FOLL_LONGTERM), to
pin_longterm_pages(), which sets both FOLL_LONGTERM and FOLL_PIN.

Note that this is a change in behavior, because the
get_user_pages_remote() call was not setting FOLL_LONGTERM, but the
new pin_user_pages_remote() call that replaces it, *is* setting
FOLL_LONGTERM. It is important to set FOLL_LONGTERM, because the
DMA case requires it. Please see the FOLL_PIN documentation in
include/linux/mm.h, and Documentation/pin_user_pages.rst for details.

2. Because all FOLL_PIN-acquired pages must be released via
put_user_page(), also convert the put_page() call over to
put_user_pages().

Note that this effectively changes the code's behavior in
vfio_iommu_type1.c: put_pfn(): it now ultimately calls
set_page_dirty_lock(), instead of set_page_dirty(). This is
probably more accurate.

As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
dealing with a file backed page where we have reference on the inode it
hangs off." [1]

[1] https://lore.kernel.org/r/20190723153640.GB720@lst.de

Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/vfio/vfio_iommu_type1.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index d864277ea16f..795e13f3ef08 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -327,9 +327,8 @@ static int put_pfn(unsigned long pfn, int prot)
 {
 	if (!is_invalid_reserved_pfn(pfn)) {
 		struct page *page = pfn_to_page(pfn);
-		if (prot & IOMMU_WRITE)
-			SetPageDirty(page);
-		put_page(page);
+
+		put_user_pages_dirty_lock(&page, 1, prot & IOMMU_WRITE);
 		return 1;
 	}
 	return 0;
@@ -349,11 +348,11 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
 
 	down_read(&mm->mmap_sem);
 	if (mm == current->mm) {
-		ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
-				     vmas);
+		ret = pin_longterm_pages(vaddr, 1, flags, page, vmas);
 	} else {
-		ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
-					    vmas, NULL);
+		ret = pin_longterm_pages_remote(NULL, mm, vaddr, 1,
+						flags, page, vmas,
+						NULL);
 		/*
 		 * The lifetime of a vaddr_get_pfn() page pin is
 		 * userspace-controlled. In the fs-dax case this could
@@ -363,7 +362,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
 		 */
 		if (ret > 0 && vma_is_fsdax(vmas[0])) {
 			ret = -EOPNOTSUPP;
-			put_page(page[0]);
+			put_user_page(page[0]);
 		}
 	}
 	up_read(&mm->mmap_sem);
-- 
2.23.0