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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