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Commit 2bb6d283 authored by Dan Williams's avatar Dan Williams Committed by Linus Torvalds
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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
V4L2).

[akpm@linux-foundation.org: use kcalloc()]
Link: http://lkml.kernel.org/r/151068939435.7446.13560129395419350737.stgit@dwillia2-desk3.amr.corp.intel.com


Fixes: 3565fce3 ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
Suggested-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Hal Rosenstock <hal.rosenstock@gmail.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9702cffd

include/linux/fs.h

+14 −0
Original line number Diff line number Diff line
@@ -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;

include/linux/mm.h

+13 −0
Original line number Diff line number Diff line
@@ -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);

#ifdef CONFIG_FS_DAX

long get_user_pages_longterm(unsigned long start, unsigned long nr_pages,

			    unsigned int gup_flags, struct page **pages,

			    struct vm_area_struct **vmas);

#else

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);

mm/gup.c

+64 −0
Original line number Diff line number Diff line
@@ -1095,6 +1095,70 @@ long get_user_pages(unsigned long start, unsigned long nr_pages, 
}

EXPORT_SYMBOL(get_user_pages);



#ifdef CONFIG_FS_DAX

/*

 * 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 *),

			       GFP_KERNEL);

		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)

			continue;



		vma_prev = vma;



		if (vma_is_fsdax(vma))

			break;

	}



	/*

	 * 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++)

		put_page(pages[i]);

	rc = -EOPNOTSUPP;

out:

	if (vmas != vmas_arg)

		kfree(vmas);

	return rc;

}

EXPORT_SYMBOL(get_user_pages_longterm);

#endif /* CONFIG_FS_DAX */



/**

 * populate_vma_page_range() -  populate a range of pages in the vma.

 * @vma:   target vma