linux-ext4 - make misbehavior on ext2 in dax mode (was: a crash when running strace from persistent memory)

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Date:   Fri, 4 Sep 2020 12:21:29 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Jann Horn <jannh@...gle.com>, Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-ext4@...r.kernel.org
Subject: make misbehavior on ext2 in dax mode (was: a crash when running
 strace from persistent memory)

On Thu, 3 Sep 2020, Mikulas Patocka wrote:

> Hi
> 
> There's a bug when you run strace from dax-based filesystem.

Hmm, so I've found another bug in dax mode.

If you extract the Linux kernel tree on dax-based ext2 filesystem (use the 
real ext2 driver, not ext4), and then you run make twice, the second 
invocation will rebuild everything. It seems like a problem with 
timestamps.

mount -t ext2 -o dax /dev/pmem0 /mnt/ext2/
cd /mnt/ext2/usr/src/git/linux-2.6
make clean
make -j12
make -j12	<--- this rebuilds the whole tree, althought it shouldn't

I wasn't able to bisect it because this bug seems to be present in every 
kernel I tried (back to 4.16.0). Ext4 doesn't seem to have this bug.

Mikulas