On Mon, 18 Oct 2010 14:58:59 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, 19 Oct 2010 00:15:04 +0800 > Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > > > Neil find that if too_many_isolated() returns true while performing > > direct reclaim we can end up waiting for other threads to complete their > > direct reclaim. If those threads are allowed to enter the FS or IO to > > free memory, but this thread is not, then it is possible that those > > threads will be waiting on this thread and so we get a circular > > deadlock. > > > > some task enters direct reclaim with GFP_KERNEL > > => too_many_isolated() false > > => vmscan and run into dirty pages > > => pageout() > > => take some FS lock > > => fs/block code does GFP_NOIO allocation > > => enter direct reclaim again > > => too_many_isolated() true > > => waiting for others to progress, however the other > > tasks may be circular waiting for the FS lock.. > > > > The fix is to let !__GFP_IO and !__GFP_FS direct reclaims enjoy higher > > priority than normal ones, by honouring them higher throttle threshold. > > > > Now !GFP_IOFS reclaims won't be waiting for GFP_IOFS reclaims to > > progress. They will be blocked only when there are too many concurrent > > !GFP_IOFS reclaims, however that's very unlikely because the IO-less > > direct reclaims is able to progress much more faster, and they won't > > deadlock each other. The threshold is raised high enough for them, so > > that there can be sufficient parallel progress of !GFP_IOFS reclaims. > > I'm not sure that this is really a full fix. Torsten's analysis does > appear to point at the real bug: raid1 has code paths which allocate > more than a single element from a mempool without starting IO against > previous elements. ... point at "a" real bug. I think there are two bugs here. The raid1 bug that Torsten mentions is certainly real (and has been around for an embarrassingly long time). The bug that I identified in too_many_isolated is also a real bug and can be triggered without md/raid1 in the mix. So this is not a 'full fix' for every bug in the kernel :-), but it could well be a full fix for this particular bug. NeilBrown > > Giving these allocations the ability to dip further into reserves will > make occurrence of the bug less likely, but if enough threads all do > this at the same time, that reserve will be exhausted and we're back to > square one? > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxxx For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>