On Tue, Jun 13, 2017 at 10:22:43AM +0530, Aneesh Kumar K.V wrote: > Ram Pai <[email protected]> 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.
ok. A v2 version of the patch series will be out in a day or two. RP
