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

[NeilBrown <neilb@suse.com>]

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On Tue, May 23 2017, Vlastimil Babka wrote:

> 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/

Yes please!!!

I particularly like:

> - GFP_KERNEL | __GFP_NORETRY - overrides the default allocator behavior and
>   all allocation requests fail early rather than cause disruptive
>   reclaim (one round of reclaim in this implementation). The OOM killer
>   is not invoked.
> - GFP_KERNEL | __GFP_RETRY_MAYFAIL - overrides the default allocator behavior
>   and all allocation requests try really hard. The request will fail if the
>   reclaim cannot make any progress. The OOM killer won't be triggered.
> - GFP_KERNEL | __GFP_NOFAIL - overrides the default allocator behavior
>   and all allocation requests will loop endlessly until they
>   succeed. This might be really dangerous especially for larger orders.

There seems to be a good range here, and the two end points are good
choices.
I like that only __GFP_NOFAIL triggers the OOM.
I would like the middle option to be the default.  I think that is what
many people thought the default was.  I appreciate that making the
transition might be awkward.
Maybe create GFP_DEFAULT which matches the middle option and encourage
that in new code??

We would probably want guidelines on when __GFP_NOFAIL is acceptable.
I assume:
  - no locks held
  - small allocations OK, large allocation need clear justification.
  - error would be exposed to systemcall
???

I think it is important to give kernel developers clear options and make
it easy for them to choose the best option.  This helps to do that.

Thanks,
NeilBrown


>
>> 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@kernel.org
>>> 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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>> 

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