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

[John Hubbard <jhubbard@nvidia.com>]

On 4/6/20 12:01 AM, Michal Hocko wrote:
...
>  From 6c90b0a19a07c87d24ad576e69b33c6e19c2f9a2 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Wed, 1 Apr 2020 14:00:56 +0200
> Subject: [PATCH] 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
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>   include/linux/gfp.h | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index e5b817cb86e7..9cacef1a3ee0 100644
> --- a/include/linux/gfp.h
> +++ b/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.
>    *
>    * %__GFP_NOMEMALLOC is used to explicitly forbid access to emergency reserves.
>    * This takes precedence over the %__GFP_MEMALLOC flag if both are set.
> 

Hi Michal and all,

How about using approximately this wording instead? I found Neil's wording to be
especially helpful so I mixed it in. (Also fixed a couple of slight 80-col overruns.)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index be2754841369..c247a911d8c7 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -111,6 +111,15 @@ struct vm_area_struct;
   * 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).
   *
+ * To be extra clear: users of __GFP_MEMALLOC must be working to free other
+ * memory, and that other memory needs to be freed "soon"; specifically, before
+ * the reserve is exhausted. This generally implies a throttling mechanism that
+ * balances the amount of __GFP_MEMALLOC memory used against the amount that the
+ * caller is about to free.
+ *
+ * Usage of a pre-allocated pool (e.g. mempool) should be always considered
+ * before using this flag.
+ *
   * %__GFP_NOMEMALLOC is used to explicitly forbid access to emergency reserves.
   * This takes precedence over the %__GFP_MEMALLOC flag if both are set.
   */


thanks,
-- 
John Hubbard
NVIDIA