Messages in this thread | | | Subject | Re: [PATCH 1/2] mm: clarify __GFP_MEMALLOC usage | From | John Hubbard <> | Date | Tue, 14 Apr 2020 12:05:57 -0700 |
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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. >> _
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