linux-kernel - [PATCH 1/1] vfio/spapr_tce: convert get_user_pages() --> pin_user

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Date:   Fri, 22 May 2020 18:43:47 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     LKML <linux-kernel@...r.kernel.org>
CC:     Souptick Joarder <jrdr.linux@...il.com>,
        John Hubbard <jhubbard@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, <kvm@...r.kernel.org>
Subject: [PATCH 1/1] vfio/spapr_tce: convert get_user_pages() --> pin_user_pages()

This code was using get_user_pages*(), in a "Case 2" scenario
(DMA/RDMA), using the categorization from [1]. That means that it's
time to convert the get_user_pages*() + put_page() calls to
pin_user_pages*() + unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Cc: Alex Williamson <alex.williamson@...hat.com>
Cc: Cornelia Huck <cohuck@...hat.com>
Cc: kvm@...r.kernel.org
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---

Hi,

I'm compile-tested this, but am not able to run-time test, so any
testing help is much appreciated!

thanks,
John Hubbard
NVIDIA

 drivers/vfio/vfio_iommu_spapr_tce.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
index 16b3adc508db..fe888b5dcc00 100644
--- a/drivers/vfio/vfio_iommu_spapr_tce.c
+++ b/drivers/vfio/vfio_iommu_spapr_tce.c
@@ -383,7 +383,7 @@ static void tce_iommu_unuse_page(struct tce_container *container,
 	struct page *page;
 
 	page = pfn_to_page(hpa >> PAGE_SHIFT);
-	put_page(page);
+	unpin_user_page(page);
 }
 
 static int tce_iommu_prereg_ua_to_hpa(struct tce_container *container,
@@ -486,7 +486,7 @@ static int tce_iommu_use_page(unsigned long tce, unsigned long *hpa)
 	struct page *page = NULL;
 	enum dma_data_direction direction = iommu_tce_direction(tce);
 
-	if (get_user_pages_fast(tce & PAGE_MASK, 1,
+	if (pin_user_pages_fast(tce & PAGE_MASK, 1,
 			direction != DMA_TO_DEVICE ? FOLL_WRITE : 0,
 			&page) != 1)
 		return -EFAULT;
-- 
2.26.2