From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Williamson Subject: Re: [PATCH 3/3] vfio: disable filesystem-dax page pinning Date: Mon, 5 Feb 2018 14:44:22 -0700 Message-ID: <20180205144422.1ca67ab5@w520.home> References: <151778551496.7139.17808629759104553625.stgit@dwillia2-desk3.amr.corp.intel.com> <151778553083.7139.6601964812589807125.stgit@dwillia2-desk3.amr.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Haozhong Zhang , Michal Hocko , jack@suse.cz, kvm@vger.kernel.org, linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, linux-fsdevel@vger.kernel.org, hch@lst.de, To: Dan Williams Return-path: In-Reply-To: <151778553083.7139.6601964812589807125.stgit@dwillia2-desk3.amr.corp.intel.com> Sender: stable-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Sun, 04 Feb 2018 15:05:30 -0800 Dan Williams wrote: > Filesystem-DAX is incompatible with 'longterm' page pinning. Without > page cache indirection a DAX mapping maps filesystem blocks directly. > This means that the filesystem must not modify a file's block map while > any page in a mapping is pinned. In order to prevent the situation of > userspace holding of filesystem operations indefinitely, disallow > 'longterm' Filesystem-DAX mappings. > > RDMA has the same conflict and the plan there is to add a 'with lease' > mechanism to allow the kernel to notify userspace that the mapping is > being torn down for block-map maintenance. Perhaps something similar can > be put in place for vfio. > > Note that xfs and ext4 still report: > > "DAX enabled. Warning: EXPERIMENTAL, use at your own risk" > > ...at mount time, and resolving the dax-dma-vs-truncate problem is one > of the last hurdles to remove that designation. > > Cc: Alex Williamson > Cc: Michal Hocko > Cc: Christoph Hellwig > Cc: kvm@vger.kernel.org > Cc: > Reported-by: Haozhong Zhang > Fixes: d475c6346a38 ("dax,ext2: replace XIP read and write with DAX I/O") > Signed-off-by: Dan Williams > --- > drivers/vfio/vfio_iommu_type1.c | 18 +++++++++++++++--- > 1 file changed, 15 insertions(+), 3 deletions(-) This isn't without some expense, a vfio mapping and un-mapping unit test incurs ~1.5% increase in system time losing access to gup_fast(). Also, I think tce_iommu_use_page() is going to have the same problem, it provides the same sort of functionality for a different vfio IOMMU backend. Please take this through your tree and I'll add a todo list item to see how we might improve this. Acked-by: Alex Williamson Thanks, Alex > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c > index e30e29ae4819..45657e2b1ff7 100644 > --- a/drivers/vfio/vfio_iommu_type1.c > +++ b/drivers/vfio/vfio_iommu_type1.c > @@ -338,11 +338,12 @@ 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]; > int ret; > > if (mm == current->mm) { > - ret = get_user_pages_fast(vaddr, 1, !!(prot & IOMMU_WRITE), > - page); > + ret = get_user_pages_longterm(vaddr, 1, !!(prot & IOMMU_WRITE), > + page, vmas); > } else { > unsigned int flags = 0; > > @@ -351,7 +352,18 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, > > down_read(&mm->mmap_sem); > ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page, > - NULL, NULL); > + 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); > } > >