Re: [PATCH 5/5] fstests: add stress truncation + writeback test

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jun 11, 2024 at 11:15:52AM -0700, Luis Chamberlain wrote:
> On Tue, Jun 11, 2024 at 07:45:03AM -0700, Darrick J. Wong wrote:
> > On Mon, Jun 10, 2024 at 08:02:02PM -0700, Luis Chamberlain wrote:
> > > +# Requires CONFIG_DEBUGFS and truncation knobs
> > > +_require_split_debugfs()
> > 
> > Er... I thought "split" referred to debugfs itself.
> > 
> > _require_split_huge_pages_knob?
> 
> Much better, thanks.
> 
> > > +# This aims at trying to reproduce a difficult to reproduce bug found with
> > > +# min order. The issue was root caused to an xarray bug when we split folios
> > > +# to another order other than 0. This functionality is used to support min
> > > +# order. The crash:
> > > +#
> > > +# https://gist.github.com/mcgrof/d12f586ec6ebe32b2472b5d634c397df
> > 
> > You might want to paste the stacktrace in here directly, in case the
> > gist ever goes away.
> 
> Its not a simple crash trace, it is pretty enourmous considering I
> decoded it, and it has all locking candidates. Even including it after
> the "---" lines of the patch might make someone go: TLDR. Thoughts?

I'd paste it in, even if it's quite lengthy.  I don't even think it's all that
much if you remove some of the less useful bits of the unwind:

"Crash excerpt is as follows:

"BUG: kernel NULL pointer dereference, address: 0000000000000036
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 7 PID: 2190 Comm: kworker/u38:5 Not tainted 6.9.0-rc5+ #14
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Workqueue: writeback wb_workfn (flush-7:5)
RIP: 0010:filemap_get_folios_tag+0xa9/0x200
Call Trace:
 <TASK>
 writeback_iter+0x17d/0x310
 write_cache_pages+0x42/0xa0
 iomap_writepages+0x33/0x50
 xfs_vm_writepages+0x63/0x90 [xfs]
 do_writepages+0xcc/0x260
 __writeback_single_inode+0x3d/0x340
 writeback_sb_inodes+0x1ed/0x4b0
 __writeback_inodes_wb+0x4c/0xe0
 wb_writeback+0x267/0x2d0
 wb_workfn+0x2a4/0x440
 process_one_work+0x189/0x3b0
 worker_thread+0x273/0x390
 kthread+0xda/0x110
 ret_from_fork+0x2d/0x50
 ret_from_fork_asm+0x1a/0x30
 </TASK>"

--D

> > > +if grep -q thp_split_page /proc/vmstat; then
> > > +	split_count_after=$(grep ^thp_split_page /proc/vmstat | head -1 | awk '{print $2}')
> > > +	split_count_failed_after=$(grep ^thp_split_page_failed /proc/vmstat | head -1 | awk '{print $2}')
> > 
> > I think this ought to be a separate function for cleanliness?
> > 
> > _proc_vmstat()
> > {
> > 	awk -v name="$1" '{if ($1 ~ name) {print($2)}}' /proc/vmstat
> > }
> 
> > Otherwise this test looks fine to me.
> 
> Thanks!
> 
>   Luis
>