• Dan Williams's avatar
    mm: introduce get_user_pages_longterm · 2bb6d283
    Dan Williams authored
    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: 's avatarDan Williams <dan.j.williams@intel.com>
    Suggested-by: 's 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: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    2bb6d283