Re: [PATCH/RFC] Shared page tables

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> Here's a new version of my shared page tables patch.
> 
> The primary purpose of sharing page tables is improved performance for
> large applications that share big memory areas between multiple processes.
> It eliminates the redundant page tables and significantly reduces the
> number of minor page faults.  Tests show significant performance
> improvement for large database applications, including those using large
> pages.  There is no measurable performance degradation for small processes.

Tried to get this running with CONFIG_PTSHARE and CONFIG_PTSHARE_PTE on
s390x. Unfortunately it crashed on boot, because pt_share_pte
returned a broken pte pointer:

> +pte_t *pt_share_pte(struct vm_area_struct *vma, unsigned long address, pmd_t *pmd,
> + ...
> +	pmd_val(spmde) = 0;
> + ...
> +		if (pmd_present(spmde)) {

This is wrong. A pmd_val of 0 will make pmd_present return true on s390x
which is not what you want.
Should be pmd_clear(&spmde).

> +pmd_t *pt_share_pmd(struct vm_area_struct *vma, unsigned long address, pud_t *pud,
> + ...
> +	pud_val(spude) = 0;

Should be pud_clear, I guess :)

Thanks,
Heiko
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