Benjamin Herrenschmidt <b...@kernel.crashing.org> writes: > On Wed, 2013-06-19 at 00:16 +0530, Aneesh Kumar K.V wrote: > >> But will that by anonymous memory ? ie, will we find them suitable for >> THP allocation ? > > The 4k pages themselves with 4k_PFN no, but the segment yes. A single of > these will demote the whole segment, ie 256M or 1T. > >> > * If you find a THP in hash_page and the segment size is 4k, fault >> > >> > * In do_page_fault, re-check for that condition (or maybe we can make >> > hash_page return a specific bit that gets ORed into the error_code into >> > do_page_fault ?) and split huge pages there. >> > >> > But that's just an idea off the top of my mind, there might be a better >> > way. Of course this needs to be tested. >> > >> > BTW. For the subpage protection, similarily, you need to make sure you >> > properly map the entire segment as "no THP", not just the range >> > passed-in by the user. >> >> Can you explain that more, why should the entire segment be marked no THP ? >> The segment can work with 4K base page size and we still be able to >> allocate a hugepage in that segment. > > Will we be able to track all the possible hashings of the huge page on a > 4k segment ?
Yes. The comment above hpte_valid explains that * The linux hugepage PMD now include the pmd entries followed by the address * to the stashed pgtable_t. The stashed pgtable_t contains the hpte bits. * [ 1 bit secondary | 3 bit hidx | 1 bit valid | 000]. We use one byte per * each HPTE entry. With 16MB hugepage and 64K HPTE we need 256 entries and * with 4K HPTE we need 4096 entries. Both will fit in a 4K pgtable_t. -aneesh _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
