Subject: Re: [PATCH 3/3] powerpc: mm: optimize for the correctly placed page

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 05 Dec 2013 16:28:27 +0530

Liu Ping Fan <[email-masked]> writes:

> The period check of _PAGE_NUMA can probably trigger the check on
> the correctly placed page. For this case, we can just insert hpte and
> do fast exception return.

I still don't understand why we need to handle numa faults in hash
page ? Are you trying to optimize the code path ? If so can you explain
the benefits ? Some numbers showing it is helping  ?

>
> Signed-off-by: Liu Ping Fan <[email-masked]>
> ---
>  arch/powerpc/mm/hash_utils_64.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
> index 9bf1195..735678c 100644
> --- a/arch/powerpc/mm/hash_utils_64.c
> +++ b/arch/powerpc/mm/hash_utils_64.c
> @@ -965,6 +965,10 @@ int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
>  	const struct cpumask *tmp;
>  	int rc, user_region = 0, local = 0;
>  	int psize, ssize;
> +	pte_t old, new;
> +	struct vm_area_struct *vma;
> +	int page_nid, target_nid;
> +	struct page *test_page;
>
>  	DBG_LOW("hash_page(ea=%016lx, access=%lx, trap=%lx\n",
>  		ea, access, trap);
> @@ -1033,12 +1037,40 @@ int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
>
>  	/* Get PTE and page size from page tables */
>  	ptep = find_linux_pte_or_hugepte(pgdir, ea, &hugeshift);
> -	if (ptep == NULL || !pte_present(*ptep) || pte_numa(*ptep)) {
> +	if (ptep == NULL || !pte_present(*ptep)) {
>  		DBG_LOW(" no PTE !\n");
>  		rc = 1;
>  		goto bail;
>  	}
>
> +	old = pte_val(*ptep);
> +	if (pte_numa(old)) {
> +		/* If fail to lock, let do_page_fault() to handle it */
> +		if (down_read_trylock(&mm->mmap_sem)) {

hmm is that something we want to do in hash_page ?

> +			vma = find_vma(mm, ea);
> +			up_read(&mm->mmap_sem);
> +			test_page = pte_page(old);
> +			page_nid = page_to_nid(test_page);
> +			target_nid = numa_migrate_prep(test_page, vma, ea,
> +						page_nid);
> +			if (target_nid < 0) {
> +				new = pte_mknonnuma(old);
> +				/* If ptep is modified under us,
> +				 * just retry the access
> +				 */
> +				if (unlikely(cmpxchg(ptep, old, new) != old)) {
> +					put_page(test_page);
> +					return 0;
> +				}
> +				put_page(test_page);
> +			}
> +		} else {
> +				put_page(test_page);
> +				rc = 1;
> +				goto bail;
> +		}
> +	}
> +
>  	/* Add _PAGE_PRESENT to the required access perm */
>  	access |= _PAGE_PRESENT;
>

-aneesh