Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()

[Bjorn Helgaas]

On Thu, Sep 16, 2021 at 05:40:59PM -0600, Logan Gunthorpe wrote:
> Introduce pci_mmap_p2pmem() which is a helper to allocate and mmap
> a hunk of p2pmem into userspace.
> 
> Pages are allocated from the genalloc in bulk and their reference count
> incremented. They are returned to the genalloc when the page is put.
> 
> The VMA does not take a reference to the pages when they are inserted
> with vmf_insert_mixed() (which is necessary for zone device pages) so
> the backing P2P memory is stored in a structures in vm_private_data.
> 
> A pseudo mount is used to allocate an inode for each PCI device. The
> inode's address_space is used in the file doing the mmap so that all
> VMAs are collected and can be unmapped if the PCI device is unbound.
> After unmapping, the VMAs are iterated through and their pages are
> put so the device can continue to be unbound. An active flag is used
> to signal to VMAs not to allocate any further P2P memory once the
> removal process starts. The flag is synchronized with concurrent
> access with an RCU lock.
> 
> The VMAs and inode will survive after the unbind of the device, but no
> pages will be present in the VMA and a subsequent access will result
> in a SIGBUS error.
> 
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>

Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>

I would capitalize "Introduce" in the subject line.

> ---
>  drivers/pci/p2pdma.c       | 263 ++++++++++++++++++++++++++++++++++++-
>  include/linux/pci-p2pdma.h |  11 ++
>  include/uapi/linux/magic.h |   1 +
>  3 files changed, 273 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 2422af5a529c..a5adf57af53a 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -16,14 +16,19 @@
>  #include <linux/genalloc.h>
>  #include <linux/memremap.h>
>  #include <linux/percpu-refcount.h>
> +#include <linux/pfn_t.h>
> +#include <linux/pseudo_fs.h>
>  #include <linux/random.h>
>  #include <linux/seq_buf.h>
>  #include <linux/xarray.h>
> +#include <uapi/linux/magic.h>
>  
>  struct pci_p2pdma {
>  	struct gen_pool *pool;
>  	bool p2pmem_published;
>  	struct xarray map_types;
> +	struct inode *inode;
> +	bool active;
>  };
>  
>  struct pci_p2pdma_pagemap {
> @@ -32,6 +37,14 @@ struct pci_p2pdma_pagemap {
>  	u64 bus_offset;
>  };
>  
> +struct pci_p2pdma_map {
> +	struct kref ref;
> +	struct pci_dev *pdev;
> +	struct inode *inode;
> +	void *kaddr;
> +	size_t len;
> +};
> +
>  static struct pci_p2pdma_pagemap *to_p2p_pgmap(struct dev_pagemap *pgmap)
>  {
>  	return container_of(pgmap, struct pci_p2pdma_pagemap, pgmap);
> @@ -100,6 +113,26 @@ static const struct attribute_group p2pmem_group = {
>  	.name = "p2pmem",
>  };
>  
> +/*
> + * P2PDMA internal mount
> + * Fake an internal VFS mount-point in order to allocate struct address_space
> + * mappings to remove VMAs on unbind events.
> + */
> +static int pci_p2pdma_fs_cnt;
> +static struct vfsmount *pci_p2pdma_fs_mnt;
> +
> +static int pci_p2pdma_fs_init_fs_context(struct fs_context *fc)
> +{
> +	return init_pseudo(fc, P2PDMA_MAGIC) ? 0 : -ENOMEM;
> +}
> +
> +static struct file_system_type pci_p2pdma_fs_type = {
> +	.name = "p2dma",
> +	.owner = THIS_MODULE,
> +	.init_fs_context = pci_p2pdma_fs_init_fs_context,
> +	.kill_sb = kill_anon_super,
> +};
> +
>  static void p2pdma_page_free(struct page *page)
>  {
>  	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(page->pgmap);
> @@ -128,6 +161,9 @@ static void pci_p2pdma_release(void *data)
>  	gen_pool_destroy(p2pdma->pool);
>  	sysfs_remove_group(&pdev->dev.kobj, &p2pmem_group);
>  	xa_destroy(&p2pdma->map_types);
> +
> +	iput(p2pdma->inode);
> +	simple_release_fs(&pci_p2pdma_fs_mnt, &pci_p2pdma_fs_cnt);
>  }
>  
>  static int pci_p2pdma_setup(struct pci_dev *pdev)
> @@ -145,17 +181,32 @@ static int pci_p2pdma_setup(struct pci_dev *pdev)
>  	if (!p2p->pool)
>  		goto out;
>  
> -	error = devm_add_action_or_reset(&pdev->dev, pci_p2pdma_release, pdev);
> +	error = simple_pin_fs(&pci_p2pdma_fs_type, &pci_p2pdma_fs_mnt,
> +			      &pci_p2pdma_fs_cnt);
>  	if (error)
>  		goto out_pool_destroy;
>  
> +	p2p->inode = alloc_anon_inode(pci_p2pdma_fs_mnt->mnt_sb);
> +	if (IS_ERR(p2p->inode)) {
> +		error = -ENOMEM;
> +		goto out_unpin_fs;
> +	}
> +
> +	error = devm_add_action_or_reset(&pdev->dev, pci_p2pdma_release, pdev);
> +	if (error)
> +		goto out_put_inode;
> +
>  	error = sysfs_create_group(&pdev->dev.kobj, &p2pmem_group);
>  	if (error)
> -		goto out_pool_destroy;
> +		goto out_put_inode;
>  
>  	rcu_assign_pointer(pdev->p2pdma, p2p);
>  	return 0;
>  
> +out_put_inode:
> +	iput(p2p->inode);
> +out_unpin_fs:
> +	simple_release_fs(&pci_p2pdma_fs_mnt, &pci_p2pdma_fs_cnt);
>  out_pool_destroy:
>  	gen_pool_destroy(p2p->pool);
>  out:
> @@ -163,6 +214,45 @@ static int pci_p2pdma_setup(struct pci_dev *pdev)
>  	return error;
>  }
>  
> +static void pci_p2pdma_map_free_pages(struct pci_p2pdma_map *pmap)
> +{
> +	int i;
> +
> +	if (!pmap->kaddr)
> +		return;
> +
> +	for (i = 0; i < pmap->len; i += PAGE_SIZE)
> +		put_page(virt_to_page(pmap->kaddr + i));
> +
> +	pmap->kaddr = NULL;
> +}
> +
> +static void pci_p2pdma_free_mappings(struct address_space *mapping)
> +{
> +	struct vm_area_struct *vma;
> +
> +	i_mmap_lock_write(mapping);
> +	if (RB_EMPTY_ROOT(&mapping->i_mmap.rb_root))
> +		goto out;
> +
> +	vma_interval_tree_foreach(vma, &mapping->i_mmap, 0, -1)
> +		pci_p2pdma_map_free_pages(vma->vm_private_data);
> +
> +out:
> +	i_mmap_unlock_write(mapping);
> +}
> +
> +static void pci_p2pdma_unmap_mappings(void *data)
> +{
> +	struct pci_dev *pdev = data;
> +	struct pci_p2pdma *p2pdma = rcu_dereference_protected(pdev->p2pdma, 1);
> +
> +	p2pdma->active = false;
> +	synchronize_rcu();
> +	unmap_mapping_range(p2pdma->inode->i_mapping, 0, 0, 1);
> +	pci_p2pdma_free_mappings(p2pdma->inode->i_mapping);
> +}
> +
>  /**
>   * pci_p2pdma_add_resource - add memory for use as p2p memory
>   * @pdev: the device to add the memory to
> @@ -221,6 +311,11 @@ int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar, size_t size,
>  		goto pgmap_free;
>  	}
>  
> +	error = devm_add_action_or_reset(&pdev->dev, pci_p2pdma_unmap_mappings,
> +					 pdev);
> +	if (error)
> +		goto pages_free;
> +
>  	p2pdma = rcu_dereference_protected(pdev->p2pdma, 1);
>  	error = gen_pool_add_owner(p2pdma->pool, (unsigned long)addr,
>  			pci_bus_address(pdev, bar) + offset,
> @@ -229,6 +324,7 @@ int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar, size_t size,
>  	if (error)
>  		goto pages_free;
>  
> +	p2pdma->active = true;
>  	pci_info(pdev, "added peer-to-peer DMA memory %#llx-%#llx\n",
>  		 pgmap->range.start, pgmap->range.end);
>  
> @@ -1029,3 +1125,166 @@ ssize_t pci_p2pdma_enable_show(char *page, struct pci_dev *p2p_dev,
>  	return sprintf(page, "%s\n", pci_name(p2p_dev));
>  }
>  EXPORT_SYMBOL_GPL(pci_p2pdma_enable_show);
> +
> +static struct pci_p2pdma_map *pci_p2pdma_map_alloc(struct pci_dev *pdev,
> +						   size_t len)
> +{
> +	struct pci_p2pdma_map *pmap;
> +
> +	pmap = kzalloc(sizeof(*pmap), GFP_KERNEL);
> +	if (!pmap)
> +		return NULL;
> +
> +	kref_init(&pmap->ref);
> +	pmap->pdev = pci_dev_get(pdev);
> +	pmap->len = len;
> +
> +	return pmap;
> +}
> +
> +static void pci_p2pdma_map_free(struct kref *ref)
> +{
> +	struct pci_p2pdma_map *pmap =
> +		container_of(ref, struct pci_p2pdma_map, ref);
> +
> +	pci_p2pdma_map_free_pages(pmap);
> +	pci_dev_put(pmap->pdev);
> +	iput(pmap->inode);
> +	simple_release_fs(&pci_p2pdma_fs_mnt, &pci_p2pdma_fs_cnt);
> +	kfree(pmap);
> +}
> +
> +static void pci_p2pdma_vma_open(struct vm_area_struct *vma)
> +{
> +	struct pci_p2pdma_map *pmap = vma->vm_private_data;
> +
> +	kref_get(&pmap->ref);
> +}
> +
> +static void pci_p2pdma_vma_close(struct vm_area_struct *vma)
> +{
> +	struct pci_p2pdma_map *pmap = vma->vm_private_data;
> +
> +	kref_put(&pmap->ref, pci_p2pdma_map_free);
> +}
> +
> +static vm_fault_t pci_p2pdma_vma_fault(struct vm_fault *vmf)
> +{
> +	struct pci_p2pdma_map *pmap = vmf->vma->vm_private_data;
> +	struct pci_p2pdma *p2pdma;
> +	void *vaddr;
> +	pfn_t pfn;
> +	int i;
> +
> +	if (!pmap->kaddr) {
> +		rcu_read_lock();
> +		p2pdma = rcu_dereference(pmap->pdev->p2pdma);
> +		if (!p2pdma)
> +			goto err_out;
> +
> +		if (!p2pdma->active)
> +			goto err_out;
> +
> +		pmap->kaddr = (void *)gen_pool_alloc(p2pdma->pool, pmap->len);
> +		if (!pmap->kaddr)
> +			goto err_out;
> +
> +		for (i = 0; i < pmap->len; i += PAGE_SIZE)
> +			get_page(virt_to_page(pmap->kaddr + i));
> +
> +		rcu_read_unlock();
> +	}
> +
> +	vaddr = pmap->kaddr + (vmf->pgoff << PAGE_SHIFT);
> +	pfn = phys_to_pfn_t(virt_to_phys(vaddr), PFN_DEV | PFN_MAP);
> +
> +	return vmf_insert_mixed(vmf->vma, vmf->address, pfn);
> +
> +err_out:
> +	rcu_read_unlock();
> +	return VM_FAULT_SIGBUS;
> +}
> +static const struct vm_operations_struct pci_p2pdma_vmops = {
> +	.open = pci_p2pdma_vma_open,
> +	.close = pci_p2pdma_vma_close,
> +	.fault = pci_p2pdma_vma_fault,
> +};
> +
> +/**
> + * pci_p2pdma_mmap_file_open - setup file mapping to store P2PMEM VMAs
> + * @pdev: the device to allocate memory from
> + * @vma: the userspace vma to map the memory to
> + *
> + * Set f_mapping of the file to the p2pdma inode so that mappings
> + * are can be torn down on device unbind.
> + *
> + * Returns 0 on success, or a negative error code on failure
> + */
> +void pci_p2pdma_mmap_file_open(struct pci_dev *pdev, struct file *file)
> +{
> +	struct pci_p2pdma *p2pdma;
> +
> +	rcu_read_lock();
> +	p2pdma = rcu_dereference(pdev->p2pdma);
> +	if (p2pdma)
> +		file->f_mapping = p2pdma->inode->i_mapping;
> +	rcu_read_unlock();
> +}
> +EXPORT_SYMBOL_GPL(pci_p2pdma_mmap_file_open);
> +
> +/**
> + * pci_mmap_p2pmem - setup an mmap region to be backed with P2PDMA memory
> + *	that was registered with pci_p2pdma_add_resource()
> + * @pdev: the device to allocate memory from
> + * @vma: the userspace vma to map the memory to
> + *
> + * The file must call pci_p2pdma_mmap_file_open() in its open() operation.
> + *
> + * Returns 0 on success, or a negative error code on failure
> + */
> +int pci_mmap_p2pmem(struct pci_dev *pdev, struct vm_area_struct *vma)
> +{
> +	struct pci_p2pdma_map *pmap;
> +	struct pci_p2pdma *p2pdma;
> +	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;
> +	}
> +
> +	pmap = pci_p2pdma_map_alloc(pdev, vma->vm_end - vma->vm_start);
> +	if (!pmap)
> +		return -ENOMEM;
> +
> +	rcu_read_lock();
> +	p2pdma = rcu_dereference(pdev->p2pdma);
> +	if (!p2pdma) {
> +		ret = -ENODEV;
> +		goto out;
> +	}
> +
> +	ret = simple_pin_fs(&pci_p2pdma_fs_type, &pci_p2pdma_fs_mnt,
> +			    &pci_p2pdma_fs_cnt);
> +	if (ret)
> +		goto out;
> +
> +	ihold(p2pdma->inode);
> +	pmap->inode = p2pdma->inode;
> +	rcu_read_unlock();
> +
> +	vma->vm_flags |= VM_MIXEDMAP;
> +	vma->vm_private_data = pmap;
> +	vma->vm_ops = &pci_p2pdma_vmops;
> +
> +	return 0;
> +
> +out:
> +	rcu_read_unlock();
> +	kfree(pmap);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(pci_mmap_p2pmem);
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index 0c33a40a86e7..f9f19f3db676 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -81,6 +81,8 @@ int pci_p2pdma_enable_store(const char *page, struct pci_dev **p2p_dev,
>  			    bool *use_p2pdma);
>  ssize_t pci_p2pdma_enable_show(char *page, struct pci_dev *p2p_dev,
>  			       bool use_p2pdma);
> +void pci_p2pdma_mmap_file_open(struct pci_dev *pdev, struct file *file);
> +int pci_mmap_p2pmem(struct pci_dev *pdev, struct vm_area_struct *vma);
>  #else /* CONFIG_PCI_P2PDMA */
>  static inline int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar,
>  		size_t size, u64 offset)
> @@ -152,6 +154,15 @@ static inline ssize_t pci_p2pdma_enable_show(char *page,
>  {
>  	return sprintf(page, "none\n");
>  }
> +static inline void pci_p2pdma_mmap_file_open(struct pci_dev *pdev,
> +					     struct file *file)
> +{
> +}
> +static inline int pci_mmap_p2pmem(struct pci_dev *pdev,
> +				  struct vm_area_struct *vma)
> +{
> +	return -EOPNOTSUPP;
> +}
>  #endif /* CONFIG_PCI_P2PDMA */
>  
>  
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index 35687dcb1a42..af737842c56f 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -88,6 +88,7 @@
>  #define BPF_FS_MAGIC		0xcafe4a11
>  #define AAFS_MAGIC		0x5a3c69f0
>  #define ZONEFS_MAGIC		0x5a4f4653
> +#define P2PDMA_MAGIC		0x70327064
>  
>  /* Since UDF 2.01 is ISO 13346 based... */
>  #define UDF_SUPER_MAGIC		0x15013346
> -- 
> 2.30.2
>