Re: [PATCH] mm: Fix comment for NODEMASK_ALLOC

[Andrew Morton]

On Mon, 20 Aug 2018 10:55:16 +0200 Oscar Salvador <osalvador@xxxxxxxxxxxxxxxxxx> wrote:

> From: Oscar Salvador <osalvador@xxxxxxx>
> 
> Currently, NODEMASK_ALLOC allocates a nodemask_t with kmalloc when
> NODES_SHIFT is higher than 8, otherwise it declares it within the stack.
> 
> The comment says that the reasoning behind this, is that nodemask_t will be
> 256 bytes when NODES_SHIFT is higher than 8, but this is not true.
> For example, NODES_SHIFT = 9 will give us a 64 bytes nodemask_t.
> Let us fix up the comment for that.
> 
> Another thing is that it might make sense to let values lower than 128bytes
> be allocated in the stack.
> Although this all depends on the depth of the stack
> (and this changes from function to function), I think that 64 bytes
> is something we can easily afford.
> So we could even bump the limit by 1 (from > 8 to > 9).
> 

I agree.  Such a change will reduce the amount of testing which the
kmalloc version receives, but I assume there are enough people out
there testing with large NODES_SHIFT values.

And while we're looking at this, it would be nice to make NODES_SHIFT
go away.  Ensure that CONFIG_NODES_SHIFT always has a setting and use
that directly.