Re: [RFC][PATCH] allow bigger PAGE_OFFSET with PAE

William Lee Irwin III
2003-01-08T22:36:13Z

>>>> Also, this gets the kernel's pagetables right, but neglects
>>>> userspace's for now. pgd_alloc() needs to be fixed to allocate
>>>> another PMD, if the split isn't PMD-alighed.

William Lee Irwin III wrote:
>>> Um, that should be automatic when USER_PTRS_PER_PGD is increased.

On Wed, Jan 08, 2003 at 01:04:23PM -0800, Dave Hansen wrote:
>> Nope, you need a little bit more. pgd_alloc() relies on its memcpy()
>> to provide the kernel mappings. After the last user PMD is allocated,
>> you still need to copy the kernel-shared part of it in.

On Wed, Jan 08, 2003 at 02:05:33PM -0800, William Lee Irwin III wrote:
> See the bit about rounding up. Then again, the pmd entries don't get
> filled in by any of that...

Okay, basically:

#define __USER_PTRS_PER_PGD (TASK_SIZE/PGDIR_SIZE)
#define PARTIAL_PGD (TASK_SIZE > __USER_PTRS_PER_PGD*PGDIR_SIZE ? 1 : 0)
#define PARTIAL_PMD ((TASK_SIZE % PGDIR_SIZE)/PMD_SIZE)
#define USER_PTRS_PER_PGD (PARTIAL_PGD + __USER_PTRS_PER_PGD)

then

pgd_t *pgd_alloc(struct mm_struct *mm)
{
int i;
pgd_t *pgd = kmem_cache_alloc(pae_pgd_cachep, GFP_KERNEL);

if (pgd) {
for (i = 0; i < USER_PTRS_PER_PGD; i++) {
unsigned long pmd = __get_free_page(GFP_KERNEL);
if (!pmd)
goto out_oom;
clear_page(pmd);
set_pgd(pgd + i, __pgd(1 + __pa(pmd)));
}

if (USER_PTRS_PER_PGD < PTRS_PER_PGD)
memcpy(pgd + USER_PTRS_PER_PGD,
swapper_pg_dir + USER_PTRS_PER_PGD,
(PTRS_PER_PGD-USER_PTRS_PER_PGD)*sizeof(pgd_t));

if (PARTIAL_PGD) {
pgd_t *kpgd, *upgd;
pmd_t *kpmd, *upmd;

kpgd = pgd_offset_k(TASK_SIZE);
upgd = pgd_offset(mm, TASK_SIZE);
kpmd = pmd_offset(kpgd, TASK_SIZE);
upmd = pmd_offset(upgd, TASK_SIZE);

memcpy(upmd, kpmd, (PTRS_PER_PMD-PARTIAL_PMD)*sizeof(pmd_t));

}
}
return pgd;
out_oom:
for (i--; i >= 0; i--)
free_page((unsigned long)__va(pgd_val(pgd[i])-1));
kmem_cache_free(pae_pgd_cachep, pgd);
return NULL;
}

etc.