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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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