• Pavel Emelyanov's avatar
    mm: soft-dirty bits for user memory changes tracking · 0f8975ec
    Pavel Emelyanov authored
    
    
    The soft-dirty is a bit on a PTE which helps to track which pages a task
    writes to.  In order to do this tracking one should
    
      1. Clear soft-dirty bits from PTEs ("echo 4 > /proc/PID/clear_refs)
      2. Wait some time.
      3. Read soft-dirty bits (55'th in /proc/PID/pagemap2 entries)
    
    To do this tracking, the writable bit is cleared from PTEs when the
    soft-dirty bit is.  Thus, after this, when the task tries to modify a
    page at some virtual address the #PF occurs and the kernel sets the
    soft-dirty bit on the respective PTE.
    
    Note, that although all the task's address space is marked as r/o after
    the soft-dirty bits clear, the #PF-s that occur after that are processed
    fast.  This is so, since the pages are still mapped to physical memory,
    and thus all the kernel does is finds this fact out and puts back
    writable, dirty and soft-dirty bits on the PTE.
    
    Another thing to note, is that when mremap moves PTEs they are marked
    with soft-dirty as well, since from the user perspective mremap modifies
    the virtual memory at mremap's new address.
    Signed-off-by: default avatarPavel Emelyanov <xemul@parallels.com>
    Cc: Matt Mackall <mpm@selenic.com>
    Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
    Cc: Glauber Costa <glommer@parallels.com>
    Cc: Marcelo Tosatti <mtosatti@redhat.com>
    Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
    Cc: Stephen Rothwell <sfr@canb.auug.org.au>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0f8975ec
proc.txt 79 KB