On Mon, Nov 11, 2019 at 4:07 PM John Hubbard jhubbard@nvidia.com 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 jglisse@redhat.com Cc: Ira Weiny ira.weiny@intel.com 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(-)
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);
Hmm, what's the point of passing FOLL_LONGTERM to get_user_pages_remote() if get_user_pages_remote() is not going to check the vma? I think we got to this code state because the get_user_pages() vs get_user_pages_remote() split predated the introduction of FOLL_LONGTERM.
I think check_vma_flags() should do the ((FOLL_LONGTERM | FOLL_GET) && vma_is_fsdax()) check and that would also remove the need for __gup_longterm_locked.