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



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