Re: Hang / zombie process from Xarray page-fault conversion (bisected)

On Mon, Mar 11, 2019 at 9:38 PM Dave Chinner <david(a)fromorbit.com&gt; wrote: > > On Mon, Mar 11, 2019 at 08:35:05PM -0700, Dan Williams wrote: > > On Mon, Mar 11, 2019 at 8:10 AM Matthew Wilcox <willy(a)infradead.org&gt; wrote: > > > > > > On Thu, Mar 07, 2019 at 10:16:17PM -0800, Dan Williams wrote: > > > > Hi Willy, > > > > > > > > We're seeing a case where RocksDB hangs and becomes defunct when > > > > trying to kill the process. v4.19 succeeds and v4.20 fails. Robert was > > > > able to bisect this to commit b15cd800682f "dax: Convert page fault > > > > handlers to XArray". > > > > > > > > I see some direct usage of xa_index and wonder if there are some more > > > > pmd fixups to do? > > > > > > > > Other thoughts? > > > > > > I don't see why killing a process would have much to do with PMD > > > misalignment. The symptoms (hanging on a signal) smell much more like > > > leaving a locked entry in the tree. Is this easy to reproduce? Can you > > > get /proc/$pid/stack for a hung task? > > > > It's fairly easy to reproduce, I'll see if I can package up all the > > dependencies into something that fails in a VM. > > > > It's limited to xfs, no failure on ext4 to date. > > > > The hung process appears to be: > > > > kworker/53:1-xfs-sync/pmem0 > > That's completely internal to XFS. Every 30s the work is triggered > and it either does a log flush (if the fs is active) or it syncs the > superblock to clean the log and idle the filesystem. It has nothing > to do with user processes, and I don't see why killing a process has > any effect on what it does... > > > ...and then the rest of the database processes grind to a halt from there. > > > > Robert was kind enough to capture /proc/$pid/stack, but nothing interesting: > > > > [<0>] worker_thread+0xb2/0x380 > > [<0>] kthread+0x112/0x130 > > [<0>] ret_from_fork+0x1f/0x40 > > [<0>] 0xffffffffffffffff > > Much more useful would be: > > # echo w > /proc/sysrq-trigger > > And post the entire output of dmesg. Here it is: https://gist.github.com/djbw/ca7117023305f325aca6f8ef30e11556