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    ksm: let shared pages be swappable · 5ad64688
    Hugh Dickins 提交于
    Initial implementation for swapping out KSM's shared pages: add
    page_referenced_ksm() and try_to_unmap_ksm(), which rmap.c calls when
    faced with a PageKsm page.
    
    Most of what's needed can be got from the rmap_items listed from the
    stable_node of the ksm page, without discovering the actual vma: so in
    this patch just fake up a struct vma for page_referenced_one() or
    try_to_unmap_one(), then refine that in the next patch.
    
    Add VM_NONLINEAR to ksm_madvise()'s list of exclusions: it has always been
    implicit there (being only set with VM_SHARED, already excluded), but
    let's make it explicit, to help justify the lack of nonlinear unmap.
    
    Rely on the page lock to protect against concurrent modifications to that
    page's node of the stable tree.
    
    The awkward part is not swapout but swapin: do_swap_page() and
    page_add_anon_rmap() now have to allow for new possibilities - perhaps a
    ksm page still in swapcache, perhaps a swapcache page associated with one
    location in one anon_vma now needed for another location or anon_vma.
    (And the vma might even be no longer VM_MERGEABLE when that happens.)
    
    ksm_might_need_to_copy() checks for that case, and supplies a duplicate
    page when necessary, simply leaving it to a subsequent pass of ksmd to
    rediscover the identity and merge them back into one ksm page.
    Disappointingly primitive: but the alternative would have to accumulate
    unswappable info about the swapped out ksm pages, limiting swappability.
    
    Remove page_add_ksm_rmap(): page_add_anon_rmap() now has to allow for the
    particular case it was handling, so just use it instead.
    Signed-off-by: NHugh Dickins <hugh.dickins@tiscali.co.uk>
    Cc: Izik Eidus <ieidus@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Chris Wright <chrisw@redhat.com>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    5ad64688
ksm.c 47.4 KB