Re: Subject: [PATCH] mm: use vm_map_ram for only temporal object

[SeongJae Park]

Hello Gioh,

On Mon, Mar 10, 2014 at 7:16 PM, Gioh Kim <gioh.kim@xxxxxxx> wrote:
>
> The vm_map_ram has fragment problem because it couldn't
> purge a chunk(ie, 4M address space) if there is a pinning object in
> that addresss space. So it could consume all VMALLOC address space
> easily.
> We can fix the fragmentation problem with using vmap instead of vm_map_ram
> but vmap is known to slow operation compared to vm_map_ram. Minchan said
> vm_map_ram is 5 times faster than vmap in his experiment. So I thought
> we should fix fragment problem of vm_map_ram because our proprietary
> GPU driver has used it heavily.
>
> On second thought, it's not an easy because we should reuse freed
> space for solving the problem and it could make more IPI and bitmap operation
> for searching hole. It could mitigate API's goal which is very fast mapping.
> And even fragmentation problem wouldn't show in 64 bit machine.
>
> Another option is that the user should separate long-life and short-life
> object and use vmap for long-life but vm_map_ram for short-life.
> If we inform the user about the characteristic of vm_map_ram
> the user can choose one according to the page lifetime.
>
> Let's add some notice messages to user.
>
> Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
> Reviewed-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
> ---
>  mm/vmalloc.c |    6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 0fdf968..85b6687 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1083,6 +1083,12 @@ EXPORT_SYMBOL(vm_unmap_ram);
>   * @node: prefer to allocate data structures on this node
>   * @prot: memory protection to use. PAGE_KERNEL for regular RAM
>   *
> + * If you use this function for below VMAP_MAX_ALLOC pages, it could be faster
> + * than vmap so it's good. But if you mix long-life and short-life object
> + * with vm_map_ram, it could consume lots of address space by fragmentation
> + * (expecially, 32bit machine). You could see failure in the end.

looks like trivial typo. Shouldn't s/expecially/especially/ ?

Thanks.

> + * Please use this function for short-life object.
> + *
>   * Returns: a pointer to the address that has been mapped, or %NULL on failure
>   */
>  void *vm_map_ram(struct page **pages, unsigned int count, int node, pgprot_t prot)
> --
> 1.7.9.5
>
>
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