On Aug 5, 2015, at 3:51 AM, mhocko@xxxxxxxxxx wrote: > Hi, > small GFP_NOFS, like GFP_KERNEL, allocations have not been not failing > traditionally even though their reclaim capabilities are restricted > because the VM code cannot recurse into filesystems to clean dirty > pages. At the same time these allocation requests do not allow to > trigger the OOM killer because that would lead to pre-mature OOM killing > during heavy fs metadata workloads. > > This leaves the VM code in an unfortunate situation where GFP_NOFS > requests is looping inside the allocator relying on somebody else to > make a progress on its behalf. This is prone to deadlocks when the > request is holding resources which are necessary for other task to make > a progress and release memory (e.g. OOM victim is blocked on the lock > held by the NONFS request). Another drawback is that the caller of > the allocator cannot define any fallback strategy because the request > doesn't fail. > > As the VM cannot do much about these requests we should face the reality > and allow those allocations to fail. Johannes has already posted the > patch which does that (http://marc.info/?l=linux-mm&m=142726428514236&w=2) > but the discussion died pretty quickly. > > I was playing with this patch and xfs, ext[34] and btrfs for a while > to see what is the effect under heavy memory pressure. As expected > this led to some fallouts. > > My test consisted of a simple memory hog which allocates a lot of > anonymous memory and writes to a fs mainly to trigger a fs activity on > exit. In parallel there is a parallel fs metadata load (multiple tasks > creating thousands of empty files and directories). All is running > in a VM with small amount of memory to emulate an under provisioned > system. The metadata load is triggering a sufficient load to invoke > the direct reclaim even without the memory hog. The memory hog forks > several tasks sharing the VM and OOM killer manages to kill it without > locking up the system (this was based on the test case from Tetsuo > Handa - http://www.spinics.net/lists/linux-fsdevel/msg82958.html - > I just didn't want to kill my machine ;)). > > With all the patches applied none of the 4 filesystems gets aborted > transactions and RO remount (well xfs didn't need any special > treatment). This is obviously not sufficient to claim that failing > GFP_NOFS is OK now but I think it is a good start for the further > discussion. I would be grateful if FS people could have a look at > those patches. I have simply used __GFP_NOFAIL in the critical paths. > This might be not the best strategy but it sounds like a good first > step. > > The first patch in the series also allows __GFP_NOFAIL allocations to > access memory reserves when the system is OOM which should help those > requests to make a forward progress - especially in combination with > GFP_NOFS. > > The second patch tries to address a potential pre-mature OOM killer > from the page fault path. I have posted it separately but it didn't > get much traction. > > The third patch allows GFP_NOFS to fail and I believe it should see > much more testing coverage. It would be really great if it could sit > in the mmotm tree for few release cycles so that we can catch more > fallouts. > > The rest are the FS specific patches to fortify allocations > requests which are really needed to finish transactions without RO > remounts. There might be more needed but my test case survives with > these in place. Wouldn't it make more sense to order the fs-specific patches _before_ the "GFP_NOFS can fail" patch (#3), so that once that patch is applied all known failures have already been fixed? Otherwise it could show test failures during bisection that would be confusing. Cheers, Andreas > They would obviously need some rewording if they are going to be > applied even without Patch3 and I will do that if respective > maintainers will take them. Ext3 and JBD are going away soon so they > might be dropped but they have been in the tree while I was testing > so I've kept them. > > Thoughts? Opinions? > > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html Cheers, Andreas -- 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
