Re: [PATCH 1/2] mm: clarify __GFP_MEMALLOC usage

[John Hubbard <jhubbard@nvidia.com>]

On 2020-04-13 20:56, NeilBrown wrote:
> On Mon, Apr 13 2020, Andrew Morton wrote:
> 
>> I've rather lost the plot with this little patch.  Is the below
>> suitable, or do we think that changes are needed?
>>

I recall we were trying to talk Neil into adding some of his writings
into Documentation/core-api/memory-allocation.rst, and then refer to
that from here. But that would be a separate patch I think.


thanks,
-- 
John Hubbard
NVIDIA

>>
>> From: Michal Hocko <mhocko@suse.com>
>> Subject: mm: clarify __GFP_MEMALLOC usage
>>
>> It seems that the existing documentation is not explicit about the
>> expected usage and potential risks enough.  While it is calls out that
>> users have to free memory when using this flag it is not really apparent
>> that users have to careful to not deplete memory reserves and that they
>> should implement some sort of throttling wrt.  freeing process.
>>
>> This is partly based on Neil's explanation [1].
>>
>> Let's also call out that a pre allocated pool allocator should be
>> considered.
>>
>> [1] http://lkml.kernel.org/r/877dz0yxoa.fsf@notabene.neil.brown.name
>>
>> [akpm@linux-foundation.org: coding style fixes]
>> Link: http://lkml.kernel.org/r/20200403083543.11552-2-mhocko@kernel.org
>> Signed-off-by: Michal Hocko <mhocko@suse.com>
>> Cc: David Rientjes <rientjes@google.com>
>> Cc: Joel Fernandes <joel@joelfernandes.org>
>> Cc: Neil Brown <neilb@suse.de>
>> Cc: Paul E. McKenney <paulmck@kernel.org>
>> Cc: John Hubbard <jhubbard@nvidia.com>
>> [mhocko@kernel.org: update]
>>    Link: http://lkml.kernel.org/r/20200406070137.GC19426@dhcp22.suse.cz
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> ---
>>
>>   include/linux/gfp.h |    5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> --- a/include/linux/gfp.h~mm-clarify-__gfp_memalloc-usage
>> +++ a/include/linux/gfp.h
>> @@ -110,6 +110,11 @@ struct vm_area_struct;
>>    * the caller guarantees the allocation will allow more memory to be freed
>>    * very shortly e.g. process exiting or swapping. Users either should
>>    * be the MM or co-ordinating closely with the VM (e.g. swap over NFS).
>> + * Users of this flag have to be extremely careful to not deplete the reserve
>> + * completely and implement a throttling mechanism which controls the
>> + * consumption of the reserve based on the amount of freed memory.
>> + * Usage of a pre-allocated pool (e.g. mempool) should be always considered
>> + * before using this flag.
> 
> I particularly don't like the connection between the consumption and the
> amount freed.  I don't think that say anything useful and it misses the
> main point which, I think, is having a bound on total usage.
> 
> Nichal's previous proposal is, I think, the best concrete proposal so
> far.
> 
> NeilBrown
> 
>>    *
>>    * %__GFP_NOMEMALLOC is used to explicitly forbid access to emergency reserves.
>>    * This takes precedence over the %__GFP_MEMALLOC flag if both are set.
>> _