On Fri, 29 Jun 2018 11:31:50 -0600
Jason Gunthorpe <jgg@xxxxxxxxxxxx> wrote:
> The patch noted in the fixes below converted get_user_pages_fast() to
> get_user_pages_longterm(), however the two calls differ in a few ways.
>
> First _fast() is documented to not require the mmap_sem, while _longterm()
> is documented to need it. Hold the mmap sem as required.
>
> Second, _fast accepts an 'int write' while _longterm uses 'unsigned int
> gup_flags', so the expression '!!(prot & IOMMU_WRITE)' is only working by
> luck as FOLL_WRITE is currently == 0x1. Use the expected FOLL_WRITE
> constant instead.
>
> Fixes: 94db151dc892 ("vfio: disable filesystem-dax page pinning")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> ---
> drivers/vfio/vfio_iommu_type1.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
Minor change as shown below, we don't need both branches coming up with
the FOLL_WRITE flag in slightly different ways.
> I noticed this while trying to review some RDMA code that was touching
> our get_user_pages_longterm() call site and wanted to see what others
> are doing..
>
> If someone can explain that get_user_pages_longterm() is safe to call
> without the mmap_sem held I'd love to here it!
Me too :-\
> The comments in gup.c do seem to pretty clearly state the
> __get_user_pages_locked() called internally by
> get_user_pages_longterm() needs mmap_sem held..
>
> This is confusing me because this is the only
> get_user_pages_longterm() callsite that doesn't hold the mmap_sem, and
> if it really isn't required I'd like to remove it from the RDMA code
> as well :)
commit 0e81a8fc0411c9baec88f3f65154285fede473f6
Author: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Date: Fri Jun 29 11:31:50 2018 -0600
vfio: Use get_user_pages_longterm correctly
The patch noted in the fixes below converted get_user_pages_fast() to
get_user_pages_longterm(), however the two calls differ in a few ways.
First _fast() is documented to not require the mmap_sem, while _longterm()
is documented to need it. Hold the mmap sem as required.
Second, _fast accepts an 'int write' while _longterm uses 'unsigned int
gup_flags', so the expression '!!(prot & IOMMU_WRITE)' is only working by
luck as FOLL_WRITE is currently == 0x1. Use the expected FOLL_WRITE
constant instead.
Fixes: 94db151dc892 ("vfio: disable filesystem-dax page pinning")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 2c75b33db4ac..3e5b17710a4f 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -343,18 +343,16 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
struct page *page[1];
struct vm_area_struct *vma;
struct vm_area_struct *vmas[1];
+ unsigned int flags = 0;
int ret;
+ if (prot & IOMMU_WRITE)
+ flags |= FOLL_WRITE;
+
+ down_read(&mm->mmap_sem);
if (mm == current->mm) {
- ret = get_user_pages_longterm(vaddr, 1, !!(prot & IOMMU_WRITE),
- page, vmas);
+ ret = get_user_pages_longterm(vaddr, 1, flags, page, vmas);
} else {
- unsigned int flags = 0;
-
- if (prot & IOMMU_WRITE)
- flags |= FOLL_WRITE;
-
- down_read(&mm->mmap_sem);
ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
vmas, NULL);
/*
@@ -368,8 +366,8 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
ret = -EOPNOTSUPP;
put_page(page[0]);
}
- up_read(&mm->mmap_sem);
}
+ up_read(&mm->mmap_sem);
if (ret == 1) {
*pfn = page_to_pfn(page[0]);
--
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