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  • Christoph Lameter's avatar
    Generic Virtual Memmap support for SPARSEMEM · 8f6aac41
    Christoph Lameter authored
    
    
    SPARSEMEM is a pretty nice framework that unifies quite a bit of code over all
    the arches.  It would be great if it could be the default so that we can get
    rid of various forms of DISCONTIG and other variations on memory maps.  So far
    what has hindered this are the additional lookups that SPARSEMEM introduces
    for virt_to_page and page_address.  This goes so far that the code to do this
    has to be kept in a separate function and cannot be used inline.
    
    This patch introduces a virtual memmap mode for SPARSEMEM, in which the memmap
    is mapped into a virtually contigious area, only the active sections are
    physically backed.  This allows virt_to_page page_address and cohorts become
    simple shift/add operations.  No page flag fields, no table lookups, nothing
    involving memory is required.
    
    The two key operations pfn_to_page and page_to_page become:
    
       #define __pfn_to_page(pfn)      (vmemmap + (pfn))
       #define __page_to_pfn(page)     ((page) - vmemmap)
    
    By having a virtual mapping for the memmap we allow simple access without
    wasting physical memory.  As kernel memory is typically already mapped 1:1
    this introduces no additional overhead.  The virtual mapping must be big
    enough to allow a struct page to be allocated and mapped for all valid
    physical pages.  This vill make a virtual memmap difficult to use on 32 bit
    platforms that support 36 address bits.
    
    However, if there is enough virtual space available and the arch already maps
    its 1-1 kernel space using TLBs (f.e.  true of IA64 and x86_64) then this
    technique makes SPARSEMEM lookups even more efficient than CONFIG_FLATMEM.
    FLATMEM needs to read the contents of the mem_map variable to get the start of
    the memmap and then add the offset to the required entry.  vmemmap is a
    constant to which we can simply add the offset.
    
    This patch has the potential to allow us to make SPARSMEM the default (and
    even the only) option for most systems.  It should be optimal on UP, SMP and
    NUMA on most platforms.  Then we may even be able to remove the other memory
    models: FLATMEM, DISCONTIG etc.
    
    [apw@shadowen.org: config cleanups, resplit code etc]
    [kamezawa.hiroyu@jp.fujitsu.com: Fix sparsemem_vmemmap init]
    [apw@shadowen.org: vmemmap: remove excess debugging]
    [apw@shadowen.org: simplify initialisation code and reduce duplication]
    [apw@shadowen.org: pull out the vmemmap code into its own file]
    Signed-off-by: default avatarChristoph Lameter <clameter@sgi.com>
    Signed-off-by: default avatarAndy Whitcroft <apw@shadowen.org>
    Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
    Cc: "Luck, Tony" <tony.luck@intel.com>
    Cc: Andi Kleen <ak@suse.de>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8f6aac41