• Michal Hocko's avatar
    mm: make PR_SET_THP_DISABLE immediately active · 18600332
    Michal Hocko authored
    PR_SET_THP_DISABLE has a rather subtle semantic.  It doesn't affect any
    existing mapping because it only updated mm->def_flags which is a
    template for new mappings.
    
    The mappings created after prctl(PR_SET_THP_DISABLE) have VM_NOHUGEPAGE
    flag set.  This can be quite surprising for all those applications which
    do not do prctl(); fork() & exec() and want to control their own THP
    behavior.
    
    Another usecase when the immediate semantic of the prctl might be useful
    is a combination of pre- and post-copy migration of containers with
    CRIU.  In this case CRIU populates a part of a memory region with data
    that was saved during the pre-copy stage.  Afterwards, the region is
    registered with userfaultfd and CRIU expects to get page faults for the
    parts of the region that were not yet populated.  However, khugepaged
    collapses the pages and the expected page faults do not occur.
    
    In more general case, the prctl(PR_SET_THP_DISABLE) could be used as a
    temporary mechanism for enabling/disabling THP process wide.
    
    Implementation wise, a new MMF_DISABLE_THP flag is added.  This flag is
    tested when decision whether to use huge pages is taken either during
    page fault of at the time of THP collapse.
    
    It should be noted, that the new implementation makes PR_SET_THP_DISABLE
    master override to any per-VMA setting, which was not the case
    previously.
    
    Fixes: a0715cc2 ("mm, thp: add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE")
    Link: http://lkml.kernel.org/r/1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.comSigned-off-by: default avatarMichal Hocko <mhocko@suse.com>
    Signed-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
    Cc: Pavel Emelyanov <xemul@virtuozzo.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    18600332
sys.c 60.5 KB