Re: Hang / zombie process from Xarray page-fault conversion (bisected)
On Mon, Mar 11, 2019 at 8:10 AM Matthew Wilcox <willy(a)infradead.org> 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/$pid/stack for a hung task?
It's fairly easy to reproduce, I'll see if I can package up all the
dependencies into something that fails in a VM.
It's limited to xfs, no failure on ext4 to date.
The hung process appears to be:
kworker/53:1-xfs-sync/pmem0
...and then the rest of the database processes grind to a halt from there.
Robert was kind enough to capture /proc/$pid/stack, but nothing interesting:
[<0>] worker_thread+0xb2/0x380
[<0>] kthread+0x112/0x130
[<0>] ret_from_fork+0x1f/0x40
[<0>] 0xffffffffffffffff