• Martin Schwidefsky's avatar
    CONFIG_HIGHPTE vs. sub-page page tables. · 2f569afd
    Martin Schwidefsky authored
    Background: I've implemented 1K/2K page tables for s390.  These sub-page
    page tables are required to properly support the s390 virtualization
    instruction with KVM.  The SIE instruction requires that the page tables
    have 256 page table entries (pte) followed by 256 page status table entries
    (pgste).  The pgstes are only required if the process is using the SIE
    instruction.  The pgstes are updated by the hardware and by the hypervisor
    for a number of reasons, one of them is dirty and reference bit tracking.
    To avoid wasting memory the standard pte table allocation should return
    1K/2K (31/64 bit) and 2K/4K if the process is using SIE.
    Problem: Page size on s390 is 4K, page table size is 1K or 2K.  That means
    the s390 version for pte_alloc_one cannot return a pointer to a struct
    page.  Trouble is that with the CONFIG_HIGHPTE feature on x86 pte_alloc_one
    cannot return a pointer to a pte either, since that would require more than
    32 bit for the return value of pte_alloc_one (and the pte * would not be
    accessible since its not kmapped).
    Solution: The only solution I found to this dilemma is a new typedef: a
    pgtable_t.  For s390 pgtable_t will be a (pte *) - to be introduced with a
    later patch.  For everybody else it will be a (struct page *).  The
    additional problem with the initialization of the ptl lock and the
    NR_PAGETABLE accounting is solved with a constructor pgtable_page_ctor and
    a destructor pgtable_page_dtor.  The page table allocation and free
    functions need to call these two whenever a page table page is allocated or
    freed.  pmd_populate will get a pgtable_t instead of a struct page pointer.
     To get the pgtable_t back from a pmd entry that has been installed with
    pmd_populate a new function pmd_pgtable is added.  It replaces the pmd_page
    call in free_pte_range and apply_to_pte_range.
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: <linux-arch@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
vmalloc.c 20 KB