Commit 2bb6d283 authored by Dan Williams's avatar Dan Williams Committed by Linus Torvalds

mm: introduce get_user_pages_longterm

Patch series "introduce get_user_pages_longterm()", v2.

Here is a new get_user_pages api for cases where a driver intends to
keep an elevated page count indefinitely.  This is distinct from usages
like iov_iter_get_pages where the elevated page counts are transient.
The iov_iter_get_pages cases immediately turn around and submit the
pages to a device driver which will put_page when the i/o operation
completes (under kernel control).

In the longterm case userspace is responsible for dropping the page
reference at some undefined point in the future.  This is untenable for
filesystem-dax case where the filesystem is in control of the lifetime
of the block / page and needs reasonable limits on how long it can wait
for pages in a mapping to become idle.

Fixing filesystems to actually wait for dax pages to be idle before
blocks from a truncate/hole-punch operation are repurposed is saved for
a later patch series.

Also, allowing longterm registration of dax mappings is a future patch
series that introduces a "map with lease" semantic where the kernel can
revoke a lease and force userspace to drop its page references.

I have also tagged these for -stable to purposely break cases that might
assume that longterm memory registrations for filesystem-dax mappings
were supported by the kernel.  The behavior regression this policy
change implies is one of the reasons we maintain the "dax enabled.
Warning: EXPERIMENTAL, use at your own risk" notification when mounting
a filesystem in dax mode.

It is worth noting the device-dax interface does not suffer the same
constraints since it does not support file space management operations
like hole-punch.

This patch (of 4):

Until there is a solution to the dma-to-dax vs truncate problem it is
not safe to allow long standing memory registrations against
filesytem-dax vmas.  Device-dax vmas do not have this problem and are
explicitly allowed.

This is temporary until a "memory registration with layout-lease"
mechanism can be implemented for the affected sub-systems (RDMA and

[ use kcalloc()]
Fixes: 3565fce3 ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: default avatarDan Williams <>
Suggested-by: default avatarChristoph Hellwig <>
Cc: Doug Ledford <>
Cc: Hal Rosenstock <>
Cc: Inki Dae <>
Cc: Jan Kara <>
Cc: Jason Gunthorpe <>
Cc: Jeff Moyer <>
Cc: Joonyoung Shim <>
Cc: Kyungmin Park <>
Cc: Mauro Carvalho Chehab <>
Cc: Mel Gorman <>
Cc: Ross Zwisler <>
Cc: Sean Hefty <>
Cc: Seung-Woo Kim <>
Cc: Vlastimil Babka <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 9702cffd
......@@ -3194,6 +3194,20 @@ static inline bool vma_is_dax(struct vm_area_struct *vma)
return vma->vm_file && IS_DAX(vma->vm_file->f_mapping->host);
static inline bool vma_is_fsdax(struct vm_area_struct *vma)
struct inode *inode;
if (!vma->vm_file)
return false;
if (!vma_is_dax(vma))
return false;
inode = file_inode(vma->vm_file);
if (inode->i_mode == S_IFCHR)
return false; /* device-dax */
return true;
static inline int iocb_flags(struct file *file)
int res = 0;
......@@ -1380,6 +1380,19 @@ long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages, int *locked);
long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
struct page **pages, unsigned int gup_flags);
long get_user_pages_longterm(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas);
static inline long get_user_pages_longterm(unsigned long start,
unsigned long nr_pages, unsigned int gup_flags,
struct page **pages, struct vm_area_struct **vmas)
return get_user_pages(start, nr_pages, gup_flags, pages, vmas);
#endif /* CONFIG_FS_DAX */
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages);
......@@ -1095,6 +1095,70 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
* This is the same as get_user_pages() in that it assumes we are
* operating on the current task's mm, but it goes further to validate
* that the vmas associated with the address range are suitable for
* longterm elevated page reference counts. For example, filesystem-dax
* mappings are subject to the lifetime enforced by the filesystem and
* we need guarantees that longterm users like RDMA and V4L2 only
* establish mappings that have a kernel enforced revocation mechanism.
* "longterm" == userspace controlled elevated page count lifetime.
* Contrast this to iov_iter_get_pages() usages which are transient.
long get_user_pages_longterm(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas_arg)
struct vm_area_struct **vmas = vmas_arg;
struct vm_area_struct *vma_prev = NULL;
long rc, i;
if (!pages)
return -EINVAL;
if (!vmas) {
vmas = kcalloc(nr_pages, sizeof(struct vm_area_struct *),
if (!vmas)
return -ENOMEM;
rc = get_user_pages(start, nr_pages, gup_flags, pages, vmas);
for (i = 0; i < rc; i++) {
struct vm_area_struct *vma = vmas[i];
if (vma == vma_prev)
vma_prev = vma;
if (vma_is_fsdax(vma))
* Either get_user_pages() failed, or the vma validation
* succeeded, in either case we don't need to put_page() before
* returning.
if (i >= rc)
goto out;
for (i = 0; i < rc; i++)
if (vmas != vmas_arg)
return rc;
#endif /* CONFIG_FS_DAX */
* populate_vma_page_range() - populate a range of pages in the vma.
* @vma: target vma
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