lkml.org 
[lkml]   [2010]   [Apr]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages
    From
    On Thu, Apr 22, 2010 at 11:14 PM, Mel Gorman <mel@csn.ul.ie> wrote:
    > On Thu, Apr 22, 2010 at 07:51:53PM +0900, KAMEZAWA Hiroyuki wrote:
    >> On Thu, 22 Apr 2010 19:31:06 +0900
    >> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    >>
    >> > On Thu, 22 Apr 2010 19:13:12 +0900
    >> > Minchan Kim <minchan.kim@gmail.com> wrote:
    >> >
    >> > > On Thu, Apr 22, 2010 at 6:46 PM, KAMEZAWA Hiroyuki
    >> > > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    >> >
    >> > > > Hmm..in my test, the case was.
    >> > > >
    >> > > > Before try_to_unmap:
    >> > > >        mapcount=1, SwapCache, remap_swapcache=1
    >> > > > After remap
    >> > > >        mapcount=0, SwapCache, rc=0.
    >> > > >
    >> > > > So, I think there may be some race in rmap_walk() and vma handling or
    >> > > > anon_vma handling. migration_entry isn't found by rmap_walk.
    >> > > >
    >> > > > Hmm..it seems this kind patch will be required for debug.
    >> > >
    >>
    >> Ok, here is my patch for _fix_. But still testing...
    >> Running well at least for 30 minutes, where I can see bug in 10minutes.
    >> But this patch is too naive. please think about something better fix.
    >>
    >> ==
    >> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >>
    >> At adjust_vma(), vma's start address and pgoff is updated under
    >> write lock of mmap_sem. This means the vma's rmap information
    >> update is atoimic only under read lock of mmap_sem.
    >>
    >>
    >> Even if it's not atomic, in usual case, try_to_ummap() etc...
    >> just fails to decrease mapcount to be 0. no problem.
    >>
    >> But at page migration's rmap_walk(), it requires to know all
    >> migration_entry in page tables and recover mapcount.
    >>
    >> So, this race in vma's address is critical. When rmap_walk meet
    >> the race, rmap_walk will mistakenly get -EFAULT and don't call
    >> rmap_one(). This patch adds a lock for vma's rmap information.
    >> But, this is _very slow_.
    >
    > Ok wow. That is exceptionally well-spotted. This looks like a proper bug
    > that compaction exposes as opposed to a bug that compaction introduces.
    >
    >> We need something sophisitcated, light-weight update for this..
    >>
    >
    > In the event the VMA is backed by a file, the mapping i_mmap_lock is taken for
    > the duration of the update and is  taken elsewhere where the VMA information
    > is read such as rmap_walk_file()
    >
    > In the event the VMA is anon, vma_adjust currently talks no locks and your
    > patch introduces a new one but why not use the anon_vma lock here? Am I
    > missing something that requires the new lock?

    rmap_walk_anon doesn't hold vma's anon_vma->lock.
    It holds page->anon_vma->lock.

    --
    Kind regards,
    Minchan Kim
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2010-04-22 16:27    [W:0.037 / U:0.272 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site