Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM
From: Jason Gunthorpe <jgg@ziepe.ca>
Date: 2019-11-12 20:43:45
Also in:
bpf, dri-devel, kvm, linux-block, linux-doc, linux-fsdevel, linux-kselftest, linux-media, linux-mm, linux-rdma, linuxppc-dev, lkml
On Mon, Nov 11, 2019 at 04:06:45PM -0800, John Hubbard wrote:
As it says in the updated comment in gup.c: current FOLL_LONGTERM behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on vmas. However, the corresponding restriction in get_user_pages_remote() was slightly stricter than is actually required: it forbade all FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers that do not set the "locked" arg. Update the code and comments accordingly, and update the VFIO caller to take advantage of this, fixing a bug as a result: the VFIO caller is logically a FOLL_LONGTERM user. Thanks to Jason Gunthorpe for pointing out a clean way to fix this. Suggested-by: Jason Gunthorpe <jgg@ziepe.ca> Cc: Jerome Glisse <redacted> Cc: Ira Weiny <redacted> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++----------------- mm/gup.c | 13 ++++++++----- 2 files changed, 21 insertions(+), 22 deletions(-)
This matches what I thought, but I think DanW should check it too, and the vfio users should test..
quoted hunk ↗ jump to hunk
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index d864277ea16f..017689b7c32b 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c@@ -348,24 +348,20 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, flags |= FOLL_WRITE; down_read(&mm->mmap_sem); - if (mm == current->mm) { - ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page, - vmas); - } else { - ret = get_user_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 - * lead to indefinite stalls in filesystem operations. - * Disallow attempts to pin fs-dax pages via this - * interface. - */ - if (ret > 0 && vma_is_fsdax(vmas[0])) { - ret = -EOPNOTSUPP; - put_page(page[0]); - } + ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM, + page, vmas, NULL); + /* + * The lifetime of a vaddr_get_pfn() page pin is + * userspace-controlled. In the fs-dax case this could + * lead to indefinite stalls in filesystem operations. + * Disallow attempts to pin fs-dax pages via this + * interface. + */ + if (ret > 0 && vma_is_fsdax(vmas[0])) { + ret = -EOPNOTSUPP; + put_page(page[0]); }
AFAIK this chunk is redundant now as it is some hack to emulate
FOLL_LONGTERM? So vmas can be deleted too.
Also unclear why this function has this:
up_read(&mm->mmap_sem);
if (ret == 1) {
*pfn = page_to_pfn(page[0]);
return 0;
}
down_read(&mm->mmap_sem);
Jason