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: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Christoph 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b3b6a7e313e9..ea818ff739cd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -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);