• Hugh Dickins's avatar
    ksm: rmap_walk to remove_migation_ptes · e9995ef9
    Hugh Dickins authored
    A side-effect of making ksm pages swappable is that they have to be placed
    on the LRUs: which then exposes them to isolate_lru_page() and hence to
    page migration.
    
    Add rmap_walk() for remove_migration_ptes() to use: rmap_walk_anon() and
    rmap_walk_file() in rmap.c, but rmap_walk_ksm() in ksm.c.  Perhaps some
    consolidation with existing code is possible, but don't attempt that yet
    (try_to_unmap needs to handle nonlinears, but migration pte removal does
    not).
    
    rmap_walk() is sadly less general than it appears: rmap_walk_anon(), like
    remove_anon_migration_ptes() which it replaces, avoids calling
    page_lock_anon_vma(), because that includes a page_mapped() test which
    fails when all migration ptes are in place.  That was valid when NUMA page
    migration was introduced (holding mmap_sem provided the missing guarantee
    that anon_vma's slab had not already been destroyed), but I believe not
    valid in the memory hotremove case added since.
    
    For now do the same as before, and consider the best way to fix that
    unlikely race later on.  When fixed, we can probably use rmap_walk() on
    hwpoisoned ksm pages too: for now, they remain among hwpoison's various
    exceptions (its PageKsm test comes before the page is locked, but its
    page_lock_anon_vma fails safely if an anon gets upgraded).
    Signed-off-by: default avatarHugh 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: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    e9995ef9
ksm.c 52 KB