• Kirill A. Shutemov's avatar
    thp: allow mlocked THP again · e90309c9
    Kirill A. Shutemov authored
    Before THP refcounting rework, THP was not allowed to cross VMA
    boundary.  So, if we have THP and we split it, PG_mlocked can be safely
    transferred to small pages.
    
    With new THP refcounting and naive approach to mlocking we can end up
    with this scenario:
     1. we have a mlocked THP, which belong to one VM_LOCKED VMA.
     2. the process does munlock() on the *part* of the THP:
          - the VMA is split into two, one of them VM_LOCKED;
          - huge PMD split into PTE table;
          - THP is still mlocked;
     3. split_huge_page():
          - it transfers PG_mlocked to *all* small pages regrardless if it
    	blong to any VM_LOCKED VMA.
    
    We probably could munlock() all small pages on split_huge_page(), but I
    think we have accounting issue already on step two.
    
    Instead of forbidding mlocked pages altogether, we just avoid mlocking
    PTE-mapped THPs and munlock THPs on split_huge_pmd().
    
    This means PTE-mapped THPs will be on normal lru lists and will be split
    under memory pressure by vmscan.  After the split vmscan will detect
    unevictable small pages and mlock them.
    
    With this approach we shouldn't hit situation like described above.
    Signed-off-by: 's avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Sasha Levin <sasha.levin@oracle.com>
    Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    Cc: Jerome Marchand <jmarchan@redhat.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: Steve Capper <steve.capper@linaro.org>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Michal Hocko <mhocko@suse.cz>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: David Rientjes <rientjes@google.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    e90309c9
mlock.c 21 KB