[RFC PATCH 0/4 v2] mm: give __GFP_REPEAT a better semantic

[Michal Hocko]

Hi,
this is a follow up for __GFP_REPEAT clean up merged in 4.7. The previous
version of this patch series was posted as an RFC
http://lkml.kernel.org/r/1465212736-14637-1-git-send-email-mhocko@xxxxxxxxxx
Since then I have reconsidered the semantic and made it a counterpart
to the __GFP_NORETRY and made it the other extreme end of the retry
logic. Both are not invoking the OOM killer so they are suitable
for allocation paths with a fallback. Also a new potential user has
emerged (kvmalloc - see patch 4). I have also renamed the flag from
__GFP_RETRY_HARD to __GFP_RETRY_MAY_FAIL as this should be more clear.

I have kept the RFC status because of the semantic change. The patch 1
is an exception because it should be merge regardless of the rest.

The main motivation for the change is that the current implementation of
__GFP_REPEAT is not very much useful.

The documentation says:
 * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
 *   _might_ fail.  This depends upon the particular VM implementation.

It just fails to mention that this is true only for large (costly) high
order which has been the case since the flag was introduced. A similar
semantic would be really helpful for smal orders as well, though,
because we have places where a failure with a specific fallback error
handling is preferred to a potential endless loop inside the page
allocator.

The earlier cleanup dropped __GFP_REPEAT usage for low (!costly) order
users so only those which might use larger orders have stayed. One user
which slipped through cracks is addressed in patch 1.

Let's rename the flag to something more verbose and use it for existing
users. Semantic for those will not change. Then implement low (!costly)
orders failure path which is hit after the page allocator is about to
invoke the oom killer. Now we have a good counterpart for __GFP_NORETRY
and finally can tell try as hard as possible without the OOM killer.

Xfs code already has an existing annotation for allocations which are
allowed to fail and we can trivially map them to the new gfp flag
because it will provide the semantic KM_MAYFAIL wants.

kvmalloc will allow also !costly high order allocations to retry hard
before falling back to the vmalloc.

The patchset is based on the current linux-next.

Shortlog
Michal Hocko (4):
      s390: get rid of superfluous __GFP_REPEAT
      mm, tree wide: replace __GFP_REPEAT by __GFP_RETRY_MAYFAIL with more useful semantic
      xfs: map KM_MAYFAIL to __GFP_RETRY_MAYFAIL
      mm: kvmalloc support __GFP_RETRY_MAYFAIL for all sizes

Diffstat
 Documentation/DMA-ISA-LPC.txt                |  2 +-
 arch/powerpc/include/asm/book3s/64/pgalloc.h |  2 +-
 arch/powerpc/kvm/book3s_64_mmu_hv.c          |  2 +-
 arch/s390/mm/pgalloc.c                       |  2 +-
 drivers/mmc/host/wbsd.c                      |  2 +-
 drivers/s390/char/vmcp.c                     |  2 +-
 drivers/target/target_core_transport.c       |  2 +-
 drivers/vhost/net.c                          |  2 +-
 drivers/vhost/scsi.c                         |  2 +-
 drivers/vhost/vsock.c                        |  2 +-
 fs/btrfs/check-integrity.c                   |  2 +-
 fs/btrfs/raid56.c                            |  2 +-
 fs/xfs/kmem.h                                | 10 +++++++++
 include/linux/gfp.h                          | 32 +++++++++++++++++++---------
 include/linux/slab.h                         |  3 ++-
 include/trace/events/mmflags.h               |  2 +-
 mm/hugetlb.c                                 |  4 ++--
 mm/internal.h                                |  2 +-
 mm/page_alloc.c                              | 14 +++++++++---
 mm/sparse-vmemmap.c                          |  4 ++--
 mm/util.c                                    | 14 ++++--------
 mm/vmalloc.c                                 |  2 +-
 mm/vmscan.c                                  |  8 +++----
 net/core/dev.c                               |  6 +++---
 net/core/skbuff.c                            |  2 +-
 net/sched/sch_fq.c                           |  2 +-
 tools/perf/builtin-kmem.c                    |  2 +-
 27 files changed, 78 insertions(+), 53 deletions(-)