Re: [PATCH v2 09/17] powerpc/mm: Add new hash_page_mm()

Michael Ellerman
2014-10-02T03:48:57Z

On Tue, 2014-30-09 at 10:34:58 UTC, Michael Neuling wrote:
> From: Ian Munsie <imunsie@au1.ibm.com>
>
> This adds a new function hash_page_mm() based on the existing hash_page().
> This version allows any struct mm to be passed in, rather than assuming
> current. This is useful for servicing co-processor faults which are not in the
> context of the current running process.

I'm not a big fan. hash_page() is already a train wreck, and this doesn't make
it any better.

> diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
> index bbdb054..0a5c8c0 100644
> --- a/arch/powerpc/mm/hash_utils_64.c
> +++ b/arch/powerpc/mm/hash_utils_64.c
> @@ -904,7 +904,7 @@ void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
> return;
> slice_set_range_psize(mm, addr, 1, MMU_PAGE_4K);
> copro_flush_all_slbs(mm);
> - if (get_paca_psize(addr) != MMU_PAGE_4K) {
> + if ((get_paca_psize(addr) != MMU_PAGE_4K) && (current->mm == mm)) {
> get_paca()->context = mm->context;
> slb_flush_and_rebolt();

This is a bit fishy.

If that mm is currently running on another cpu you just failed to update it's
paca. But I think the call to check_paca_psize() in hash_page() will save you
on that cpu.

In fact we might be able to remove that synchronisation from
demote_segment_4k() and always leave it up to check_paca_psize()?

> @@ -989,26 +989,24 @@ static void check_paca_psize(unsigned long ea, struct mm_struct *mm,
> * -1 - critical hash insertion error
> * -2 - access not permitted by subpage protection mechanism
> */
> -int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
> +int hash_page_mm(struct mm_struct *mm, unsigned long ea, unsigned long access, unsigned long trap)
> {
> enum ctx_state prev_state = exception_enter();
> pgd_t *pgdir;
> unsigned long vsid;
> - struct mm_struct *mm;
> pte_t *ptep;
> unsigned hugeshift;
> const struct cpumask *tmp;
> int rc, user_region = 0, local = 0;
> int psize, ssize;
>
> - DBG_LOW("hash_page(ea=%016lx, access=%lx, trap=%lx\n",
> - ea, access, trap);
> + DBG_LOW("%s(ea=%016lx, access=%lx, trap=%lx\n",
> + __func__, ea, access, trap);
>
> /* Get region & vsid */
> switch (REGION_ID(ea)) {
> case USER_REGION_ID:
> user_region = 1;
> - mm = current->mm;
> if (! mm) {
> DBG_LOW(" user region with no mm !\n");
> rc = 1;

What about the VMALLOC case where we do:
mm = &init_mm;

Is that what you want? It seems odd that you pass an mm to the routine, but
then potentially it ends up using a different mm after all depending on the
address.

cheers