Re: [PATCH 0/4] Remove nrexceptional tracking

["Verma, Vishal L" <vishal.l.verma@intel.com>]

On Tue, 2020-08-04 at 17:17 +0100, Matthew Wilcox (Oracle) wrote:
> We actually use nrexceptional for very little these days.  It's a
> constant
> source of pain with the THP patches because we don't know how large a
> shadow entry is, so either we have to ask the xarray how many indices
> it covers, or store that information in the shadow entry (and reduce
> the amount of other information in the shadow entry proportionally).
> While tracking down the most recent case of "evict tells me I've got
> the accounting wrong again", I wondered if it might not be simpler to
> just remove it.  So here's a patch set to do just that.  I think each
> of these patches is an improvement in isolation, but the combination
> of
> all four is larger than the sum of its parts.
> 
> I'm running xfstests on this patchset right now.  If one of the DAX
> people could try it out, that'd be fantastic.
> 
> Matthew Wilcox (Oracle) (4):
>   mm: Introduce and use page_cache_empty
>   mm: Stop accounting shadow entries
>   dax: Account DAX entries as nrpages
>   mm: Remove nrexceptional from inode

Hi Matthew,

I applied these on top of 5.8 and ran them through the nvdimm unit test
suite, and saw some test failures. The first failing test signature is:

  + umount test_dax_mnt
  ./dax-ext4.sh: line 62: 15749 Segmentation fault      umount $MNT
  FAIL dax-ext4.sh (exit status: 139)

The line is: https://github.com/pmem/ndctl/blob/master/test/dax.sh#L79
And the failing umount happens right after 'run_test', which calls this:
https://github.com/pmem/ndctl/blob/master/test/dax-pmd.c


> 
>  fs/block_dev.c          |  2 +-
>  fs/dax.c                |  8 ++++----
>  fs/inode.c              |  2 +-
>  include/linux/fs.h      |  2 --
>  include/linux/pagemap.h |  5 +++++
>  mm/filemap.c            | 15 ---------------
>  mm/truncate.c           | 19 +++----------------
>  mm/workingset.c         |  1 -
>  8 files changed, 14 insertions(+), 40 deletions(-)
> 
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