On Tue, Nov 12, 2019 at 08:26:55PM -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.
Also, remove an unnessary pair of calls that were releasing and reacquiring the mmap_sem. There is no need to avoid holding mmap_sem just in order to call page_to_pfn().
Also, move the DAX check ("if a VMA is DAX, don't allow long term pinning") from the VFIO call site, all the way into the internals of get_user_pages_remote() and __gup_longterm_locked(). That is: get_user_pages_remote() calls __gup_longterm_locked(), which in turn calls check_dax_vmas(). It's lightly explained in the comments as well.
Thanks to Jason Gunthorpe for pointing out a clean way to fix this, and to Dan Williams for helping clarify the DAX refactoring.
Suggested-by: Jason Gunthorpe jgg@ziepe.ca Cc: Dan Williams dan.j.williams@intel.com 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 | 25 ++----------------------- mm/gup.c | 27 ++++++++++++++++++++++----- 2 files changed, 24 insertions(+), 28 deletions(-)
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index d864277ea16f..7301b710c9a4 100644 +++ b/drivers/vfio/vfio_iommu_type1.c @@ -340,7 +340,6 @@ 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;
@@ -348,33 +347,13 @@ 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]);}- }
- up_read(&mm->mmap_sem);
- ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
page, NULL, NULL);
Mind the return with the lock held this needs some goto unwind
Jason