Messages in this thread | | | Date | Mon, 9 Apr 2012 15:21:20 -0700 | From | Greg Kroah-Hartman <> | Subject | Re: v3.4-rc2 out-of-memory problems (was Re: 3.4-rc1 sticks-and-crashs) |
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On Mon, Apr 09, 2012 at 03:13:00PM -0700, Colin Cross wrote: > On Mon, Apr 9, 2012 at 2:22 PM, David Rientjes <rientjes@google.com> wrote: > > On Mon, 9 Apr 2012, Linus Torvalds wrote: > > > >> The real bug is actually that those notifiers are a f*cking joke, and > >> the return value from the notifier is a mistake. > >> > >> So I personally think that the real problem is this code in > >> profile_handoff_task: > >> > >> return (ret == NOTIFY_OK) ? 1 : 0; > >> > >> and ask yourself two questions: > >> > >> - what the hell does NOTIFY_OK/NOTIFY_DONE mean? > >> - what happens if there are multiple notifiers that all (or some) > >> return NOTIFY_OK? > >> > > NOTIFY_OK should never be a valid response for this notifier the way it's > > currently implemented, it should be NOTIFY_STOP to stop iterating the call > > chain to avoid a double free. Right now it doesn't matter because only > > oprofile is actually freeing the task_struct and lowmemorykiller should be > > using NOTIFY_DONE. > > > > Then we have a completeness issue if multiple callbacks want to return > > NOTIFY_STOP and an ordering issue if the oprofile callback is invoked > > before lowmemorykiller. > > > >> I'll tell you what my answers are: > >> > >> (a) NOTIFY_DONE is the "ok, everything is fine, you can free the > >> task-struct". It's also what that handoff notifier thing returns if > >> there are no notifiers registered at all. > >> > >> So the fix to the Android lowmemorykiller is as simple as just > >> changing NOTIFY_OK to NOTIFY_DONE, which will mean that the caller > >> will properly free the task struct. > >> > > > > I don't think so for Werner's config who also has CONFIG_OPROFILE=y, so > > oprofile would return NOTIFY_OK and queue the task_struct for free, then > > the second notifier callback to the lowmemorykiller would return > > NOTIFY_DONE which would result in put_task_struct() doing free_task() > > itself for a double free. > > > >> The NOTIFY_OK/NOTIFY_DONE difference really does seem to be just > >> "NOTIFY_OK means that I will free the task myself later". That's what > >> the oprofile uses, and it frees the task. > >> > >> (b) But the whole interface is a total f*cking mess. If *multiple* > >> people return NOTIFY_OK, they're royally fucked. And the whole (and > >> only) point of notifiers is that you can register multiple different > >> ones independently. > >> > >> So quite frankly, the *real* bug is not in that android driver > >> (although I'd say that we should just make it return NOTIFY_DONE and > >> be done with it). The real bug is that the whole f*cking notifier is a > >> mistake, and checking the error return was the biggest mistake of all. > >> > > > > Right, we can't handoff the freeing of the task_struct to more than one > > notifier. It seems misdesigned from the beginning and what we really want > > is to hijack task->usage for __put_task_struct(task) if we have such a > > notifier callchain and require each one (currently just oprofile) to take > > a reference on task->usage for NOTIFY_OK and then be responsible for > > dropping the reference when it's done with it later instead of requiring > > it to free the task_struct itself. > > > > That's _if_ we want to continue to have such an interface in the first > > place where it's only really necessary right now for oprofile (and, hence, > > wasn't implemented in an extendable way). I'm thinking the > > lowmemorykiller, as I eluded to, could be written in a way where we can > > detect if a thread we've already killed has exited yet before killing > > another one. We can't just store a pointer to the task_struct of the > > killed task since it could be reused for a fork later, but we could use > > TIF_MEMDIE like the oom killer does. > > This was a known issue in 2010, in the android tree the use of > task_handoff_register was dropped one day after it was added and > replaced with a new task_free_register hook. I assume Greg dropped > the fix during the android tree refresh in 3.0 because it depended on > a change to kernel/fork.c. The two relevant patches are (using > codeaurora's gitweb becase we don't have one right now): > > sched: Add a generic notifier when a task struct is about to be freed > https://www.codeaurora.org/gitweb/quic/la/?p=kernel/common.git;a=commitdiff;h=667dffa787a87ef4ea43cc65957ce96077fdcd0a
Yes, I can't add a patch like that for this driver, that is why I thought everyone was getting together to "properly" determine how to solve this oom notifier problem. Has that work stalled somwhere?
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