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

[Vlastimil Babka]

On 05/16/2017 11:10 AM, Michal Hocko wrote:
> So, is there some interest in this? I am not going to push this if there
> is a general consensus that we do not need to do anything about the
> current situation or need a different approach.

After the recent LWN article [1] I think that we should really support
marking allocations as failable, without making them too easily failable
via __GFP_NORETRY. The __GFP_RETRY_MAY_FAIL flag sounds like a good way
to do that without introducing a new __GFP_MAYFAIL. We could also
introduce a wrapper such as GFP_KERNEL_MAYFAIL.

[1] https://lwn.net/Articles/723317/

> On Tue 07-03-17 16:48:39, Michal Hocko wrote:
>> 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.keprnel.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(-)
>>
>> --
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