Re: [RFC PATCH 1/7 v1]powerpc: Free up four PTE bits to accommodate memory keys

On Tue, Jun 13, 2017 at 10:22:43AM +0530, Aneesh Kumar K.V wrote:

Ram Pai [off-list ref] writes:

quoted

Rearrange  PTE   bits to  free  up  bits 3, 4, 5  and  6  for
memory keys. Bit 3, 4, 5, 6 and 57  shall  be used for memory
keys.

The patch does the following change to the 64K PTE format

H_PAGE_BUSY moves from bit 3 to bit 7
H_PAGE_F_SECOND which occupied bit 4 moves to the second part
	of the pte.
H_PAGE_F_GIX which  occupied bit 5, 6 and 7 also moves to the
	second part of the pte.

The second part of the PTE will hold
   a (H_PAGE_F_SECOND|H_PAGE_F_GIX)  for  64K page backed pte,
   and sixteen (H_PAGE_F_SECOND|H_PAGE_F_GIX)  for 4k  backed
	pte.

the four  bits((H_PAGE_F_SECOND|H_PAGE_F_GIX) that represent a slot
is initialized to 0xF indicating a invalid slot. if a hashpage does
get  allocated  to  the  0xF  slot, it is released and not used. In
other words, even  though  0xF  is  a valid slot we discard it  and
consider it as invalid slot(HPTE_SOFT_INVALID). This  gives  us  an
opportunity to  not  depend on a bit in the primary PTE in order to
determine the validity of a slot.

When  we  release  a  0xF slot we also release a legitimate primary
slot  and  unmap  that  entry. This  is  to  ensure  that we do get
a legimate non-0xF slot the next time we retry for a slot.

Though treating 0xF slot as invalid reduces the number of available
slots and make have a effect on the performance, the probabilty
of hitting a 0xF is extermely low.

Compared  to the current scheme, the above described scheme reduces
the number of false hash table updates  significantly  and  has the
added  advantage  of  releasing  four  valuable  PTE bits for other
purpose.

This idea was jointly developed by Paul Mackerras, Aneesh, Michael
Ellermen and myself.

4K PTE format remain unchanged currently.

Can you also split this patch into two. One which changes
__hash_page_4k() ie, linux pte format w.r.t 4k hash pte. Second patch
with changes w.r.t __hash_page_64k() ie, pte format w.r.t 64k hash pte.

ok. A v2 version of the patch series will be out in a day or two.
RP