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    SubjectRe: [PATCH -mm 2/4] oom: select_bad_process: PF_EXITING check should take ->mm into account
    On 04/06, anfei wrote:
    >
    > On Fri, Apr 02, 2010 at 08:32:16PM +0200, Oleg Nesterov wrote:
    > > select_bad_process() checks PF_EXITING to detect the task which
    > > is going to release its memory, but the logic is very wrong.
    > >
    > > - a single process P with the dead group leader disables
    > > select_bad_process() completely, it will always return
    > > ERR_PTR() while P can live forever
    > >
    > > - if the PF_EXITING task has already released its ->mm
    > > it doesn't make sense to expect it is goiing to free
    > > more memory (except task_struct/etc)
    > >
    > > Change the code to ignore the PF_EXITING tasks without ->mm.
    > >
    > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    > > ---
    > >
    > > mm/oom_kill.c | 2 +-
    > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > >
    > > --- MM/mm/oom_kill.c~2_FIX_PF_EXITING 2010-04-02 18:51:05.000000000 +0200
    > > +++ MM/mm/oom_kill.c 2010-04-02 18:58:37.000000000 +0200
    > > @@ -322,7 +322,7 @@ static struct task_struct *select_bad_pr
    > > * the process of exiting and releasing its resources.
    > > * Otherwise we could get an easy OOM deadlock.
    > > */
    > > - if (p->flags & PF_EXITING) {
    > > + if ((p->flags & PF_EXITING) && p->mm) {
    >
    > Even this check is satisfied, it still can't say p is a good victim or
    > it will release memory automatically if multi threaded, as the exiting
    > of p doesn't mean the other threads are going to exit, so the ->mm won't
    > be released.

    Yes, completely agreed.

    Unfortunately I forgot to copy this into the changelog, but when I
    discussed this change I mentioned "still not perfect, but much better".

    I do not really know what is the "right" solution. Even if we fix this
    check for mt case, we also have CLONE_VM tasks.

    So, this patch just tries to improve things, to avoid the easy-to-trigger
    false positives.

    Oleg.



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