Hi Zlatko, On Fri, Dec 28, 2012 at 02:29:11PM +0100, Zlatko Calusic wrote: > On 28.12.2012 03:49, Minchan Kim wrote: > >Hello Zlatko, > > > >On Fri, Dec 28, 2012 at 03:16:38AM +0100, Zlatko Calusic wrote: > >>From: Zlatko Calusic <zlatko.calusic@xxxxxxxx> > >> > >>The unintended consequence of commit 4ae0a48b is that > >>wait_iff_congested() can now be called with NULL struct zone* > >>producing kernel oops like this: > > > >For good description, it would be better to write simple pseudo code > >flow to show how NULL-zone pass into wait_iff_congested because > >kswapd code flow is too complex. > > > >As I see the code, we have following line above wait_iff_congested. > > > >if (!unbalanced_zone || blah blah) > > break; > > > >How can NULL unbalanced_zone reach wait_iff_congested? > > > > Hello Minchan, and thanks for the comment. > > That line was there before commit 4ae0a48b got in, and you're right, Argh, I didn't see 4ae0a48b in 3.8-rc1. > it's what was protecting wait_iff_congested() from being called with > NULL zone*. But then all that logic got colapsed to a simple > pgdat_balanced() call and that's when I introduced the bug, I lost > the protection. > > What I _think_ is happening (pseudo code following...) is that after > scanning the zone in the dma->highmem direction, and concluding that > all zones are balanced (unbalanced_zone remains NULL!), > wake_up(&pgdat->pfmemalloc_wait) wakes up a lot of memory hungry > processes (especially true in various aggressive test/benchmarks) > that immediately drain and unbalance one or more zones. Then > pgdat_balanced() call which immediately follows will be false, but > we still have unbalanced_zone = NULL, rememeber? Oops... > > But, all that is a speculation that I can't prove atm. Of course, if > anybody thinks that's a credible explanation, I could add it as a > commit comment, or even as a code comment, but I didn't want to be > overly imaginative. The fix itself is simple and real. Never mind. My confusing is caused my missing 4ae0a48b in lasest tree. Thanks, Zlatko. > > Regards, > -- > Zlatko > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>