Messages in this thread | | | Date | Mon, 28 Jan 2008 00:56:57 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] Only print kernel debug information for OOMs caused by kernel allocations |
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On Mon, 28 Jan 2008 07:10:07 +0100 Andi Kleen <ak@suse.de> wrote:
> On Monday 28 January 2008 06:52, Andrew Morton wrote: > > On Wed, 16 Jan 2008 23:24:21 +0100 Andi Kleen <ak@suse.de> wrote: > > > I recently suffered an 20+ minutes oom thrash disk to death and computer > > > completely unresponsive situation on my desktop when some user program > > > decided to grab all memory. It eventually recovered, but left lots > > > of ugly and imho misleading messages in the kernel log. here's a minor > > > improvement > > As a followup this was with swap over dm crypt. I've recently heard > about other people having trouble with this too so this setup seems to trigger > something bad in the VM.
Where's the backtrace and show_mem() output? :)
> > That information is useful for working out why a userspace allocation > > attempt failed. If we don't print it, and the application gets killed and > > thus frees a lot of memory, we will just never know why the allocation > > failed. > > But it's basically only either page fault (direct or indirect) or write et.al. > who do these page cache allocations. Do you really think it is that important > to distingush these cases individually? In 95+% of all cases it should > be a standard user page fault which always has the same backtrace.
Sure, the backtrace isn't very important. The show_mem() output is vital.
> To figure out why the application really oom'ed for those you would > need a user level backtrace, but the message doesn't supply that one anyways. > > All other cases will still print the full backtrace so if some kernel > subsystem runs amok it should be still possible to diagnose it. >
We need the show_mem() output to see where all the memory went, and to see what state page reclaim is in.
> > > > > > struct page *__page_cache_alloc(gfp_t gfp) > > > { > > > + struct task_struct *me = current; > > > + unsigned old = (~me->flags) & PF_USER_ALLOC; > > > + struct page *p; > > > + > > > + me->flags |= PF_USER_ALLOC; > > > if (cpuset_do_page_mem_spread()) { > > > int n = cpuset_mem_spread_node(); > > > - return alloc_pages_node(n, gfp, 0); > > > - } > > > - return alloc_pages(gfp, 0); > > > + p = alloc_pages_node(n, gfp, 0); > > > + } else > > > + p = alloc_pages(gfp, 0); > > > + /* Clear USER_ALLOC if it wasn't set originally */ > > > + me->flags ^= old; > > > + return p; > > > } > > > > That's appreciable amount of new overhead for at best a fairly marginal > > benefit. Perhaps __GFP_USER could be [re|ab]used. > > It's a few non atomic bit operations. You really think that is considerable > overhead? Also all should be cache hot already. My guess is that even with the > additional function call it's < 10 cycles more.
Plus an additional function call. On the already-deep page allocation path, I might add.
> > Alternatively: if we've printed the diagnostic on behalf of this process > > and then decided to kill it, set some flag to prevent us from printing it > > again. > > Do you really think that would help? I thought these messages came usually > from different processes.
Dunno.
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