Re: [PATCH 3/7] powerpc: Free up four 64K PTE bits in 4K backed HPTE pages

On Mon, Oct 23, 2017 at 02:17:39PM +0530, Aneesh Kumar K.V wrote:

Michael Ellerman [off-list ref] writes:

quoted

Ram Pai [off-list ref] writes:

quoted

diff --git a/arch/powerpc/mm/hash64_64k.c b/arch/powerpc/mm/hash64_64k.c
index 1a68cb1..c6c5559 100644
--- a/arch/powerpc/mm/hash64_64k.c
+++ b/arch/powerpc/mm/hash64_64k.c

@@ -126,18 +113,13 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
 	if (__rpte_sub_valid(rpte, subpg_index)) {
 		int ret;
 
-		hash = hpt_hash(vpn, shift, ssize);
-		hidx = __rpte_to_hidx(rpte, subpg_index);
-		if (hidx & _PTEIDX_SECONDARY)
-			hash = ~hash;
-		slot = (hash & htab_hash_mask) * HPTES_PER_GROUP;
-		slot += hidx & _PTEIDX_GROUP_IX;
+		gslot = pte_get_hash_gslot(vpn, shift, ssize, rpte,
+					subpg_index);
+		ret = mmu_hash_ops.hpte_updatepp(gslot, rflags, vpn,
+			MMU_PAGE_4K, MMU_PAGE_4K, ssize, flags);

This was formatted correctly before:
  

quoted

-		ret = mmu_hash_ops.hpte_updatepp(slot, rflags, vpn,
-						 MMU_PAGE_4K, MMU_PAGE_4K,
-						 ssize, flags);
 		/*
-		 *if we failed because typically the HPTE wasn't really here
+		 * if we failed because typically the HPTE wasn't really here

If you're fixing it up please make it "If ...".

quoted

 		 * we try an insertion.
 		 */
 		if (ret == -1)

@@ -148,6 +130,15 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
 	}
 
 htab_insert_hpte:
+
+	/*
+	 * initialize all hidx entries to invalid value,
+	 * the first time the PTE is about to allocate
+	 * a 4K hpte
+	 */

Should be:
	/*
	 * Initialize all hidx entries to invalid value, the first time
         * the PTE is about to allocate a 4K HPTE.
	 */

quoted

+	if (!(old_pte & H_PAGE_COMBO))
+		rpte.hidx = ~0x0UL;
+

Paul had the idea that if we biased the slot number by 1, we could make
the "invalid" value be == 0.

That would avoid needing to that above, and also mean the value is
correctly invalid from the get-go, which would be good IMO.

I think now that you've added the slot accessors it would be pretty easy
to do.

That would be imply, we loose one slot in primary group, which means we
will do extra work in some case because our primary now has only 7
slots. And in case of pseries, the hypervisor will always return the
least available slot, which implie we will do extra hcalls in case of an
hpte insert to an empty group?

No. that is not the idea.  The idea is that slot 'F' in the seconday
will continue to be a invalid slot, but will be represented as
offset-by-one in the PTE.  In other words, 0 will be repesented as 1,
1 as 2....   and  n as (n+1)%32

The idea seems feasible.  It has the advantage -- where 0 in the PTE
means invalid slot. But it can be confusing to the casual code-
reader. Will need to put in a big-huge comment to explain that.

RP