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    SubjectRe: [patch 12/12] mm: correctly synchronize rss-counters at exit/exec
    Hugh Dickins wrote:
    > On Thu, 7 Jun 2012, Linus Torvalds wrote:
    >
    >> Ugh, looking more at the patch, I'm getting more and more convinces
    >> that it is pure and utter garbage.
    >>
    >> It does "sync_mm_rss(mm);" in mmput(), _after_ it has done the
    >> possibly final mmdrop(). WTF?
    >>
    >> This is crap, guys. Seriously. Stop playing russian rulette with this
    >> code. I think we need to revert *all* of the crazy rss games, unless
    >> Konstantin can show us some truly obviously correct fix.
    >>
    >> Sadly, I merged and pushed out the crap before I had rebooted and
    >> noticed this problem, so now it's in the wild. Can somebody please
    >> take a look at this asap?
    >>
    >> Linus
    >>
    >> On Thu, Jun 7, 2012 at 5:17 PM, Linus Torvalds
    >> <torvalds@linux-foundation.org> wrote:
    >>> This patch actually seems to have made the
    >>>
    >>> BUG: Bad rss-counter state ..
    >>>
    >>> problem *much* worse. It triggers all the time for me now - I've got
    >>> 408 of those messages on my macbook air within a minute of booting it.
    >>>
    >>> Not good. Especially not good when it's marked for stable too.
    >
    > I'm on the Cc, but I've not been following closely, I've not been able
    > to reproduce the issue with Konstantin's commit in just now, and I've
    > not even tried Oleg's version: so, don't trust me an inch.

    Easiest way to reproduce this:

    #define _GNU_SOURCE
    #include <stdlib.h>
    #include <sched.h>
    #include <sys/mman.h>

    int child(void *arg)
    {
    return 0;
    }

    char stack[4096];

    int main(int argc, char **argv)
    {
    void *page;

    page = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
    clone(child, stack + sizeof(stack), CLONE_VFORK | CLONE_VM | CLONE_CHILD_CLEARTID, NULL, NULL, NULL, page);
    return 0;
    }

    As result you can see "BUG: Bad rss-counter state mm:ffff88040783a680 idx:1 val:-1" in dmesg

    There left only one problem: nobody calls sync_mm_rss() after put_user() in mm_release().

    My patch is really messy, so please ignore it.
    Oleg's http://marc.info/?l=linux-kernel&m=133848759505805 version much simpler and changes only sync_mm_rss() calls.

    >
    > But I was surprised that that patch went to you, I thought Konstantin
    > and Oleg had just reached agreement that Oleg's version (reposted this
    > morning in the "3.4-rc7: BUG: Bad rss-counter state" thread) was nicer.
    >
    > And it looks like it does not do anything offensive in mmput().
    > Doing things after something called "mm_release" sounds equally
    > bad, but IIRC that's not so.
    >
    > You probably want to revert Konstantin's, try out Oleg's on your Air,
    > and maybe wait for Konstantin and Oleg to confirm the below.
    >
    > Offline for a few hours,
    > Hugh
    >
    > [PATCH] correctly synchronize rss-counters at exit/exec
    >
    > From: Oleg Nesterov<oleg@redhat.com>
    >
    > A simplified version of Konstantin Khlebnikov's patch.
    >
    > do_exit() and exec_mmap() call sync_mm_rss() before mm_release()
    > does put_user(clear_child_tid) which can update task->rss_stat
    > and thus make mm->rss_stat inconsistent. This triggers the "BUG:"
    > printk in check_mm().
    >
    > - Move the final sync_mm_rss() from do_exit() to exit_mm(), and
    > change exec_mmap() to call sync_mm_rss() after mm_release() to
    > make check_mm() happy.
    >
    > Perhaps we should simply move it into mm_release() and call it
    > unconditionally to catch the "task->rss_stat != 0&& !task->mm"
    > bugs.
    >
    > - Since taskstats_exit() is called before exit_mm(), add another
    > sync_mm_rss() into xacct_add_tsk() who actually uses rss_stat.
    >
    > Probably we should also shift acct_update_integrals().
    >
    > Reported-by: Markus Trippelsdorf<markus@trippelsdorf.de>
    > Tested-by: Martin Mokrejs<mmokrejs@fold.natur.cuni.cz>
    > Signed-off-by: Oleg Nesterov<oleg@redhat.com>
    > Acked-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
    > ---
    > fs/exec.c | 2 +-
    > kernel/exit.c | 5 ++---
    > kernel/tsacct.c | 1 +
    > 3 files changed, 4 insertions(+), 4 deletions(-)
    >
    > diff --git a/fs/exec.c b/fs/exec.c
    > index 52c9e2f..e49e3c2 100644
    > --- a/fs/exec.c
    > +++ b/fs/exec.c
    > @@ -823,10 +823,10 @@ static int exec_mmap(struct mm_struct *mm)
    > /* Notify parent that we're no longer interested in the old VM */
    > tsk = current;
    > old_mm = current->mm;
    > - sync_mm_rss(old_mm);
    > mm_release(tsk, old_mm);
    >
    > if (old_mm) {
    > + sync_mm_rss(old_mm);
    > /*
    > * Make sure that if there is a core dump in progress
    > * for the old mm, we get out and die instead of going
    > diff --git a/kernel/exit.c b/kernel/exit.c
    > index ab972a7..b3a84b5 100644
    > --- a/kernel/exit.c
    > +++ b/kernel/exit.c
    > @@ -655,6 +655,8 @@ static void exit_mm(struct task_struct * tsk)
    > mm_release(tsk, mm);
    > if (!mm)
    > return;
    > +
    > + sync_mm_rss(mm);
    > /*
    > * Serialize with any possible pending coredump.
    > * We must hold mmap_sem around checking core_state
    > @@ -965,9 +967,6 @@ void do_exit(long code)
    > preempt_count());
    >
    > acct_update_integrals(tsk);
    > - /* sync mm's RSS info before statistics gathering */
    > - if (tsk->mm)
    > - sync_mm_rss(tsk->mm);
    > group_dead = atomic_dec_and_test(&tsk->signal->live);
    > if (group_dead) {
    > hrtimer_cancel(&tsk->signal->real_timer);
    > diff --git a/kernel/tsacct.c b/kernel/tsacct.c
    > index 23b4d78..a64ee90 100644
    > --- a/kernel/tsacct.c
    > +++ b/kernel/tsacct.c
    > @@ -91,6 +91,7 @@ void xacct_add_tsk(struct taskstats *stats, struct task_struct *p)
    > stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / MB;
    > mm = get_task_mm(p);
    > if (mm) {
    > + sync_mm_rss(mm);
    > /* adjust to KB unit */
    > stats->hiwater_rss = get_mm_hiwater_rss(mm) * PAGE_SIZE / KB;
    > stats->hiwater_vm = get_mm_hiwater_vm(mm) * PAGE_SIZE / KB;



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