Re: [PATCH 3/3] NOMMU: Make the initial mmap allocation excess behaviour Kconfig configurable

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 05 May 2009 22:26:48 +0100
David Howells <dhowells@redhat.com> wrote:

> NOMMU mmap() has an option controlled by a sysctl variable that determines
> whether the allocations made by do_mmap_private() should have the excess space
> trimmed off and returned to the allocator.  Make the initial setting of this
> variable a Kconfig configuration option.
> 
> The reason there can be excess space is that the allocator only allocates in
> power-of-2 size chunks, but mmap()'s can be made in sizes that aren't a power
> of 2.
> 
> There are two alternatives:
> 
>  (1) Keep the excess as dead space.  The dead space then remains unused for the
>      lifetime of the mapping.  Mappings of shared objects such as libc, ld.so
>      or busybox's text segment may retain their dead space forever.
> 
>  (2) Return the excess to the allocator.  This means that the dead space is
>      limited to less than a page per mapping, but it means that for a transient
>      process, there's more chance of fragmentation as the excess space may be
>      reused fairly quickly.
> 
> During the boot process, a lot of transient processes are created, and this can
> cause a lot of fragmentation as the pagecache and various slabs grow greatly
> during this time.
> 
> By turning off the trimming of excess space during boot and disabling batching
> of frees, Coldfire can manage to boot.
> 
> A better way of doing things might be to have /sbin/init turn this option off.
> By that point libc, ld.so and init - which are all long-duration processes -
> have all been loaded and trimmed.
> 

Nasty problem.

> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -66,7 +66,7 @@ struct percpu_counter vm_committed_as;
>  int sysctl_overcommit_memory = OVERCOMMIT_GUESS; /* heuristic overcommit */
>  int sysctl_overcommit_ratio = 50; /* default is 50% */
>  int sysctl_max_map_count = DEFAULT_MAX_MAP_COUNT;
> -int sysctl_nr_trim_pages = 1; /* page trimming behaviour */
> +int sysctl_nr_trim_pages = CONFIG_NOMMU_INITIAL_TRIM_EXCESS;
>  int heap_stack_gap = 0;
>  

But there's a risk of -ENOMEM regression on other system here?

It's unlikely to be a huge problem for real-world embedded developers,
as long as they know about this change.  And because you set the
Kconfig default to "no change" then I guess they'll be none the wiser.

I think that patches 2 and 3 (and #1 unless I reorder and redo things)
are 2.6.30 material.  Agree?