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SubjectRe: [RFC PATCH 1/7 v1]powerpc: Free up four PTE bits to accommodate memory keys
Date
Ram Pai <linuxram@us.ibm.com> writes:

> 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.

-aneesh

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