Messages in this thread Patch in this message | | | Subject | Re: [PATCH 1/2] mm: clarify __GFP_MEMALLOC usage | From | John Hubbard <> | Date | Mon, 6 Apr 2020 12:02:05 -0700 |
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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
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