• Ira Weiny's avatar
    mm/gup: replace get_user_pages_longterm() with FOLL_LONGTERM · 932f4a63
    Ira Weiny authored
    Pach series "Add FOLL_LONGTERM to GUP fast and use it".
    
    HFI1, qib, and mthca, use get_user_pages_fast() due to its performance
    advantages.  These pages can be held for a significant time.  But
    get_user_pages_fast() does not protect against mapping FS DAX pages.
    
    Introduce FOLL_LONGTERM and use this flag in get_user_pages_fast() which
    retains the performance while also adding the FS DAX checks.  XDP has also
    shown interest in using this functionality.[1]
    
    In addition we change get_user_pages() to use the new FOLL_LONGTERM flag
    and remove the specialized get_user_pages_longterm call.
    
    [1] https://lkml.org/lkml/2019/3/19/939
    
    "longterm" is a relative thing and at this point is probably a misnomer.
    This is really flagging a pin which is going to be given to hardware and
    can't move.  I've thought of a couple of alternative names but I think we
    have to settle on if we are going to use FL_LAYOUT or something else to
    solve the "longterm" problem.  Then I think we can change the flag to a
    better name.
    
    Secondly, it depends on how often you are registering memory.  I have
    spoken with some RDMA users who consider MR in the performance path...
    For the overall application performance.  I don't have the numbers as the
    tests for HFI1 were done a long time ago.  But there was a significant
    advantage.  Some of which is probably due to the fact that you don't have
    to hold mmap_sem.
    
    Finally, architecturally I think it would be good for everyone to use
    *_fast.  There are patches submitted to the RDMA list which would allow
    the use of *_fast (they reworking the use of mmap_sem) and as soon as they
    are accepted I'll submit a patch to convert the RDMA core as well.  Also
    to this point others are looking to use *_fast.
    
    As an aside, Jasons pointed out in my previous submission that *_fast and
    *_unlocked look very much the same.  I agree and I think further cleanup
    will be coming.  But I'm focused on getting the final solution for DAX at
    the moment.
    
    This patch (of 7):
    
    This patch starts a series which aims to support FOLL_LONGTERM in
    get_user_pages_fast().  Some callers who would like to do a longterm (user
    controlled pin) of pages with the fast variant of GUP for performance
    purposes.
    
    Rather than have a separate get_user_pages_longterm() call, introduce
    FOLL_LONGTERM and change the longterm callers to use it.
    
    This patch does not change any functionality.  In the short term
    "longterm" or user controlled pins are unsafe for Filesystems and FS DAX
    in particular has been blocked.  However, callers of get_user_pages_fast()
    were not "protected".
    
    FOLL_LONGTERM can _only_ be supported with get_user_pages[_fast]() as it
    requires vmas to determine if DAX is in use.
    
    NOTE: In merging with the CMA changes we opt to change the
    get_user_pages() call in check_and_migrate_cma_pages() to a call of
    __get_user_pages_locked() on the newly migrated pages.  This makes the
    code read better in that we are calling __get_user_pages_locked() on the
    pages before and after a potential migration.
    
    As a side affect some of the interfaces are cleaned up but this is not the
    primary purpose of the series.
    
    In review[1] it was asked:
    
    <quote>
    > This I don't get - if you do lock down long term mappings performance
    > of the actual get_user_pages call shouldn't matter to start with.
    >
    > What do I miss?
    
    A couple of points.
    
    First "longterm" is a relative thing and at this point is probably a
    misnomer.  This is really flagging a pin which is going to be given to
    hardware and can't move.  I've thought of a couple of alternative names
    but I think we have to settle on if we are going to use FL_LAYOUT or
    something else to solve the "longterm" problem.  Then I think we can
    change the flag to a better name.
    
    Second, It depends on how often you are registering memory.  I have spoken
    with some RDMA users who consider MR in the performance path...  For the
    overall application performance.  I don't have the numbers as the tests
    for HFI1 were done a long time ago.  But there was a significant
    advantage.  Some of which is probably due to the fact that you don't have
    to hold mmap_sem.
    
    Finally, architecturally I think it would be good for everyone to use
    *_fast.  There are patches submitted to the RDMA list which would allow
    the use of *_fast (they reworking the use of mmap_sem) and as soon as they
    are accepted I'll submit a patch to convert the RDMA core as well.  Also
    to this point others are looking to use *_fast.
    
    As an asside, Jasons pointed out in my previous submission that *_fast and
    *_unlocked look very much the same.  I agree and I think further cleanup
    will be coming.  But I'm focused on getting the final solution for DAX at
    the moment.
    
    </quote>
    
    [1] https://lore.kernel.org/lkml/20190220180255.GA12020@iweiny-DESK2.sc.intel.com/T/#md6abad2569f3bf6c1f03686c8097ab6563e94965
    
    [ira.weiny@intel.com: v3]
      Link: http://lkml.kernel.org/r/20190328084422.29911-2-ira.weiny@intel.com
    Link: http://lkml.kernel.org/r/20190328084422.29911-2-ira.weiny@intel.com
    Link: http://lkml.kernel.org/r/20190317183438.2057-2-ira.weiny@intel.comSigned-off-by: default avatarIra Weiny <ira.weiny@intel.com>
    Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: John Hubbard <jhubbard@nvidia.com>
    Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Jason Gunthorpe <jgg@ziepe.ca>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
    Cc: Rich Felker <dalias@libc.org>
    Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: James Hogan <jhogan@kernel.org>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc: Mike Marshall <hubcap@omnibond.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    932f4a63