On Tue, Jun 15, 2010 at 10:43:42AM -0400, Christoph Hellwig wrote: > Other callers of ->writepage are fine because they come from a > controlled environment with relatively little stack usage. The problem > with direct reclaim is that we splice multiple stack hogs ontop of each > other. It's not like we're doing a stack recursive algorithm in kernel. These have to be "controlled hogs", so we must have space to run 4/5 of them on top of each other, that's the whole point. I'm aware the ->writepage can run on any alloc_pages, but frankly I don't see a whole lot of difference between regular kernel code paths or msync. Sure they can be at higher stack usage, but not like with only 1000bytes left. > And seriously, if the VM isn't stopped from calling ->writepage from > reclaim context we FS people will simply ignore any ->writepage from > reclaim context. Been there, done that and never again. > > Just wondering, what filesystems do your hugepage testing systems use? > If it's any of the ext4/btrfs/xfs above you're already seeing the > filesystem refuse ->writepage from both kswapd and direct reclaim, > so Mel's series will allow us to reclaim pages from more contexts > than before. fs ignoring ->writepage during memory pressure (even from kswapd) is broken, this is not up to the fs to decide. I'm using ext4 on most of my testing, it works ok, but it doesn't make it right (if fact if performance declines without that hack, it may prove VM needs fixing, it doesn't justify the hack). If you don't throttle against kswapd, or if even kswapd can't turn a dirty page into a clean one, you can get oom false positives. Anything is better than that. (provided you've proper stack instrumentation to notice when there is risk of a stack overflow, it's ages I never seen a stack overflow debug detector report) The irq stack must be enabled and this isn't about direct reclaim but about irqs in general and their potential nesting with softirq calls too. Also note, there's nothing that prevents us from switching the stack to something else the moment we enter direct reclaim. It doesn't need to be physically contiguous. Just allocate a couple of 4k pages and switch to them every time a new hog starts in VM context. The only real complexity is in the stack unwind but if irqstack can cope with it sure stack unwind can cope with more "special" stacks too. Ignoring ->writepage on VM invocations at best can only hide VM inefficiencies with the downside of breaking the VM in corner cases with heavy VM pressure. Crippling down the kernel by vetoing ->writepage to me looks very wrong, but I'd be totally supportive of a "special" writepage stack or special iscsi stack etc... -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
