• C
    [PATCH] Swapless page migration: add R/W migration entries · 0697212a
    Christoph Lameter 提交于
    Implement read/write migration ptes
    
    We take the upper two swapfiles for the two types of migration ptes and define
    a series of macros in swapops.h.
    
    The VM is modified to handle the migration entries.  migration entries can
    only be encountered when the page they are pointing to is locked.  This limits
    the number of places one has to fix.  We also check in copy_pte_range and in
    mprotect_pte_range() for migration ptes.
    
    We check for migration ptes in do_swap_cache and call a function that will
    then wait on the page lock.  This allows us to effectively stop all accesses
    to apge.
    
    Migration entries are created by try_to_unmap if called for migration and
    removed by local functions in migrate.c
    
    From: Hugh Dickins <hugh@veritas.com>
    
      Several times while testing swapless page migration (I've no NUMA, just
      hacking it up to migrate recklessly while running load), I've hit the
      BUG_ON(!PageLocked(p)) in migration_entry_to_page.
    
      This comes from an orphaned migration entry, unrelated to the current
      correctly locked migration, but hit by remove_anon_migration_ptes as it
      checks an address in each vma of the anon_vma list.
    
      Such an orphan may be left behind if an earlier migration raced with fork:
      copy_one_pte can duplicate a migration entry from parent to child, after
      remove_anon_migration_ptes has checked the child vma, but before it has
      removed it from the parent vma.  (If the process were later to fault on this
      orphaned entry, it would hit the same BUG from migration_entry_wait.)
    
      This could be fixed by locking anon_vma in copy_one_pte, but we'd rather
      not.  There's no such problem with file pages, because vma_prio_tree_add
      adds child vma after parent vma, and the page table locking at each end is
      enough to serialize.  Follow that example with anon_vma: add new vmas to the
      tail instead of the head.
    
      (There's no corresponding problem when inserting migration entries,
      because a missed pte will leave the page count and mapcount high, which is
      allowed for.  And there's no corresponding problem when migrating via swap,
      because a leftover swap entry will be correctly faulted.  But the swapless
      method has no refcounting of its entries.)
    
    From: Ingo Molnar <mingo@elte.hu>
    
      pte_unmap_unlock() takes the pte pointer as an argument.
    
    From: Hugh Dickins <hugh@veritas.com>
    
      Several times while testing swapless page migration, gcc has tried to exec
      a pointer instead of a string: smells like COW mappings are not being
      properly write-protected on fork.
    
      The protection in copy_one_pte looks very convincing, until at last you
      realize that the second arg to make_migration_entry is a boolean "write",
      and SWP_MIGRATION_READ is 30.
    
      Anyway, it's better done like in change_pte_range, using
      is_write_migration_entry and make_migration_entry_read.
    
    From: Hugh Dickins <hugh@veritas.com>
    
      Remove unnecessary obfuscation from sys_swapon's range check on swap type,
      which blew up causing memory corruption once swapless migration made
      MAX_SWAPFILES no longer 2 ^ MAX_SWAPFILES_SHIFT.
    Signed-off-by: NHugh Dickins <hugh@veritas.com>
    Acked-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
    Signed-off-by: NHugh Dickins <hugh@veritas.com>
    Signed-off-by: NChristoph Lameter <clameter@engr.sgi.com>
    Signed-off-by: NIngo Molnar <mingo@elte.hu>
    From: Hugh Dickins <hugh@veritas.com>
    Signed-off-by: NAndrew Morton <akpm@osdl.org>
    Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
    0697212a
migrate.c 17.2 KB