Re: Shrink the size of tlb handler and fix vmalloc()

[Wu Fei <at.wufei@gmail.com>]

On Thu, Sep 03, 2009 at 10:27:53PM +0800, Wu Fei wrote:
> This patch tries to shrink the size of the 64bit tlb handler and also fix
> an vmalloc bug at the same time.
> 
Forgot to say, it's only tested on 2.6.27, not the master, I can't get
the environment to run the latest kernel currently.

Thanks,
Wufei.

> By combining the swapper_pg_dir and module_pg_dir, several checks in tlb
> handler, particularly build_get_pgd_vmalloc64, are not necessary. The reason
> they can be combined is that, the effective virtual address of vmalloc returned
> is at the bottom, and of module_alloc returned is at the top.
> 
> In the normal case of 4KB page size:
>   VMALLOC_START, VMALLOC_END	0xc0000000 00000000 - 0xc0000100 00000000
>   MODULE_START,  MODULE_END	0xffffffff c0000000 - 0xffffffff +xxxxxxx
> Change it to:
>   VMALLOC_START, VMALLOC_END	0xc0000000 00000000 - 0xc00000ff 00000000
>   MODULE_START,  MODULE_END	0xffffffff c0000000 - 0xffffffff +xxxxxxx
> We use the least 40 bits to traverse the page table, the change makes it still
> one-to-one mapping without more checking. "+" is in the range of [c,d,e,f], 
> so there even are big holes bewteen them.
> 
> With this patch, the tlb refill handler only contains about 28 instructions,
> instead of the original 38.
> 
> 
> And this patch also fix a bug in vmalloc, which happens when its returned
> address is not covered by the first pgd. e.g. if we do two vmallocs, the first
> returned address is 0xc0000000 00000000, and the 2nd is 0xc0000000 40000000,
> 
>   vmalloc -> __vmalloc_node -> __vmalloc_area_node -> __vmalloc_area_node
>   -> map_vm_area -> pgd_offset_k
> 
> pgd_offset_k doesn't use the address to index the pgd, just return the first one:
> 
>   #define pgd_offset_k(address) \ 
>         ((address) >= MODULE_START ? module_pg_dir : pgd_offset(&init_mm, 0UL))     
> 
> This is wrong, then the 2 addresses are mapped to the same pte. This bug doesn't
> happen because even in the 4KB page case, one pgd can cover 1GB size, and it looks
> like the system won't vmalloc so much area.