• Claudio Imbrenda's avatar
    mm/ksm: fix interaction with THP · 77da2ba0
    Claudio Imbrenda authored
    This patch fixes a corner case for KSM.  When two pages belong or
    belonged to the same transparent hugepage, and they should be merged,
    KSM fails to split the page, and therefore no merging happens.
    
    This bug can be reproduced by:
    * making sure ksm is running (in case disabling ksmtuned)
    * enabling transparent hugepages
    * allocating a THP-aligned 1-THP-sized buffer
      e.g. on amd64: posix_memalign(&p, 1<<21, 1<<21)
    * filling it with the same values
      e.g. memset(p, 42, 1<<21)
    * performing madvise to make it mergeable
      e.g. madvise(p, 1<<21, MADV_MERGEABLE)
    * waiting for KSM to perform a few scans
    
    The expected outcome is that the all the pages get merged (1 shared and
    the rest sharing); the actual outcome is that no pages get merged (1
    unshared and the rest volatile)
    
    The reason of this behaviour is that we increase the reference count
    once for both pages we want to merge, but if they belong to the same
    hugepage (or compound page), the reference counter used in both cases is
    the one of the head of the compound page.  This means that
    split_huge_page will find a value of the reference counter too high and
    will fail.
    
    This patch solves this problem by testing if the two pages to merge
    belong to the same hugepage when attempting to merge them.  If so, the
    hugepage is split safely.  This means that the hugepage is not split if
    not necessary.
    
    Link: http://lkml.kernel.org/r/1521548069-24758-1-git-send-email-imbrenda@linux.vnet.ibm.comSigned-off-by: 's avatarClaudio Imbrenda <imbrenda@linux.vnet.ibm.com>
    Co-authored-by: 's avatarGerald Schaefer <gerald.schaefer@de.ibm.com>
    Reviewed-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Christian Borntraeger <borntraeger@de.ibm.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    77da2ba0
ksm.c 86.8 KB