On Mon, Sep 26, 2016 at 11:27:08AM +0300, Alexander Shishkin wrote: > Peter Zijlstra <pet...@infradead.org> writes: > > > Well, we could 'force' inject a VMA into the process's address space, we > > do that for a few other things as well. It also makes for less > > exceptions with the actual core dumping. > > Threads then will end up with the same buffer (through sharing the mm), > but they can't really share trace buffers. > > Also, system core dump is still a problem.
Hurm, true on both counts. > > But the worry I have is the total amount of pinned memory. If you want > > to inherit this on fork(), as is a reasonable expectation, then its > > possible to quickly exceed the total amount of pinnable memory. > > > > At which point we _should_ start failing fork(), which is a somewhat > > unexpected, and undesirable side-effect. > > I'm not sure I see why we should fail fork() when we run out of pinned > memory. Well, we cannot fully honour the inherit, what other option do we have? Silently malfunctioning? That's far worse. > > Ideally we'd unpin the old buffers and repin the new buffers on context > > switch, but that's impossible since faulting needs scheduling, > > recursion, we loose. > > Or we can have per-cpu buffers for all user's tasks, record where each > task starts and ends in each buffer and cut out only bits relevant to > the task(s) that dump core. Which gets you the problem that when a task dumps core there might not be any state in the buffer, because the previous task flushed it all out :/
