Re: [PATCH v9 7/8] PCI/P2PDMA: Allow userspace VMA allocations through sysfs

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Sep 01, 2022 at 10:32:55AM -0600, Logan Gunthorpe wrote:
> 
> 
> 
> On 2022-09-01 10:20, Greg Kroah-Hartman wrote:
> > On Thu, Aug 25, 2022 at 09:24:24AM -0600, Logan Gunthorpe wrote:
> >> Create a sysfs bin attribute called "allocate" under the existing
> >> "p2pmem" group. The only allowable operation on this file is the mmap()
> >> call.
> >>
> >> When mmap() is called on this attribute, the kernel allocates a chunk of
> >> memory from the genalloc and inserts the pages into the VMA. The
> >> dev_pagemap .page_free callback will indicate when these pages are no
> >> longer used and they will be put back into the genalloc.
> >>
> >> On device unbind, remove the sysfs file before the memremap_pages are
> >> cleaned up. This ensures unmap_mapping_range() is called on the files
> >> inode and no new mappings can be created.
> >>
> >> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> >> ---
> >>  drivers/pci/p2pdma.c | 124 +++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 124 insertions(+)
> >>
> >> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> >> index 4496a7c5c478..a6ed6bbca214 100644
> >> --- a/drivers/pci/p2pdma.c
> >> +++ b/drivers/pci/p2pdma.c
> >> @@ -89,6 +89,90 @@ static ssize_t published_show(struct device *dev, struct device_attribute *attr,
> >>  }
> >>  static DEVICE_ATTR_RO(published);
> >>  
> >> +static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
> >> +		struct bin_attribute *attr, struct vm_area_struct *vma)
> >> +{
> >> +	struct pci_dev *pdev = to_pci_dev(kobj_to_dev(kobj));
> >> +	size_t len = vma->vm_end - vma->vm_start;
> >> +	struct pci_p2pdma *p2pdma;
> >> +	struct percpu_ref *ref;
> >> +	unsigned long vaddr;
> >> +	void *kaddr;
> >> +	int ret;
> >> +
> >> +	/* prevent private mappings from being established */
> >> +	if ((vma->vm_flags & VM_MAYSHARE) != VM_MAYSHARE) {
> >> +		pci_info_ratelimited(pdev,
> >> +				     "%s: fail, attempted private mapping\n",
> >> +				     current->comm);
> >> +		return -EINVAL;
> >> +	}
> >> +
> >> +	if (vma->vm_pgoff) {
> >> +		pci_info_ratelimited(pdev,
> >> +				     "%s: fail, attempted mapping with non-zero offset\n",
> >> +				     current->comm);
> >> +		return -EINVAL;
> >> +	}
> >> +
> >> +	rcu_read_lock();
> >> +	p2pdma = rcu_dereference(pdev->p2pdma);
> >> +	if (!p2pdma) {
> >> +		ret = -ENODEV;
> >> +		goto out;
> >> +	}
> >> +
> >> +	kaddr = (void *)gen_pool_alloc_owner(p2pdma->pool, len, (void **)&ref);
> >> +	if (!kaddr) {
> >> +		ret = -ENOMEM;
> >> +		goto out;
> >> +	}
> >> +
> >> +	/*
> >> +	 * vm_insert_page() can sleep, so a reference is taken to mapping
> >> +	 * such that rcu_read_unlock() can be done before inserting the
> >> +	 * pages
> >> +	 */
> >> +	if (unlikely(!percpu_ref_tryget_live_rcu(ref))) {
> >> +		ret = -ENODEV;
> >> +		goto out_free_mem;
> >> +	}
> >> +	rcu_read_unlock();
> >> +
> >> +	for (vaddr = vma->vm_start; vaddr < vma->vm_end; vaddr += PAGE_SIZE) {
> >> +		ret = vm_insert_page(vma, vaddr, virt_to_page(kaddr));
> >> +		if (ret) {
> >> +			gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> >> +			return ret;
> >> +		}
> >> +		percpu_ref_get(ref);
> >> +		put_page(virt_to_page(kaddr));
> >> +		kaddr += PAGE_SIZE;
> >> +		len -= PAGE_SIZE;
> >> +	}
> >> +
> >> +	percpu_ref_put(ref);
> >> +
> >> +	return 0;
> >> +out_free_mem:
> >> +	gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> >> +out:
> >> +	rcu_read_unlock();
> >> +	return ret;
> >> +}
> >> +
> >> +static struct bin_attribute p2pmem_alloc_attr = {
> >> +	.attr = { .name = "allocate", .mode = 0660 },
> >> +	.mmap = p2pmem_alloc_mmap,
> >> +	/*
> >> +	 * Some places where we want to call mmap (ie. python) will check
> >> +	 * that the file size is greater than the mmap size before allowing
> >> +	 * the mmap to continue. To work around this, just set the size
> >> +	 * to be very large.
> >> +	 */
> >> +	.size = SZ_1T,
> >> +};
> >> +
> >>  static struct attribute *p2pmem_attrs[] = {
> >>  	&dev_attr_size.attr,
> >>  	&dev_attr_available.attr,
> >> @@ -96,11 +180,32 @@ static struct attribute *p2pmem_attrs[] = {
> >>  	NULL,
> >>  };
> >>  
> >> +static struct bin_attribute *p2pmem_bin_attrs[] = {
> >> +	&p2pmem_alloc_attr,
> >> +	NULL,
> >> +};
> >> +
> >>  static const struct attribute_group p2pmem_group = {
> >>  	.attrs = p2pmem_attrs,
> >> +	.bin_attrs = p2pmem_bin_attrs,
> >>  	.name = "p2pmem",
> >>  };
> >>  
> >> +static void p2pdma_page_free(struct page *page)
> >> +{
> >> +	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(page->pgmap);
> >> +	struct percpu_ref *ref;
> >> +
> >> +	gen_pool_free_owner(pgmap->provider->p2pdma->pool,
> >> +			    (uintptr_t)page_to_virt(page), PAGE_SIZE,
> >> +			    (void **)&ref);
> >> +	percpu_ref_put(ref);
> >> +}
> >> +
> >> +static const struct dev_pagemap_ops p2pdma_pgmap_ops = {
> >> +	.page_free = p2pdma_page_free,
> >> +};
> >> +
> >>  static void pci_p2pdma_release(void *data)
> >>  {
> >>  	struct pci_dev *pdev = data;
> >> @@ -152,6 +257,19 @@ static int pci_p2pdma_setup(struct pci_dev *pdev)
> >>  	return error;
> >>  }
> >>  
> >> +static void pci_p2pdma_unmap_mappings(void *data)
> >> +{
> >> +	struct pci_dev *pdev = data;
> >> +
> >> +	/*
> >> +	 * Removing the alloc attribute from sysfs will call
> >> +	 * unmap_mapping_range() on the inode, teardown any existing userspace
> >> +	 * mappings and prevent new ones from being created.
> >> +	 */
> >> +	sysfs_remove_file_from_group(&pdev->dev.kobj, &p2pmem_alloc_attr.attr,
> >> +				     p2pmem_group.name);
> > 
> > Wait, why are you manually removing the sysfs file here?  It's part of
> > the group, if you do this then it is gone for forever, right?  Why
> > manually do this the sysfs core should handle this for you if the device
> > is removed.
> 
> We have to make sure the mappings are all removed before the cleanup of
> devm_memremap_pages() which will wait for all the pages to be freed.

Then don't use devm_ functions.  Why not just use the manual functions
instead as you know when you want to tear this down.

> If
> we don't do this any userspace mapping will hang the cleanup until those
> uses are unmapped themselves.

Just do this in the remove call yourself and you should be fine.

thanks,

greg k-h