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

[NeilBrown]

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?
>
>
> From: Michal Hocko <mhocko@xxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxxxxxxx
>
> [akpm@xxxxxxxxxxxxxxxxxxxx: coding style fixes]
> Link: http://lkml.kernel.org/r/20200403083543.11552-2-mhocko@xxxxxxxxxx
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Neil Brown <neilb@xxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> [mhocko@xxxxxxxxxx: update]
>   Link: http://lkml.kernel.org/r/20200406070137.GC19426@xxxxxxxxxxxxxx
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
>  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.
> _

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