linux-ext4 - Re: [PATCH] fstests: generic: Check if cycle mount and sleep can affect fiemap result

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Date:   Fri, 7 Apr 2017 08:42:25 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Eryu Guan <eguan@...hat.com>
Cc:     Eric Sandeen <sandeen@...deen.net>,
        "Theodore Ts'o" <tytso@....edu>,
        Qu Wenruo <quwenruo@...fujitsu.com>,
        linux-btrfs@...r.kernel.org, fstests@...r.kernel.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH] fstests: generic: Check if cycle mount and sleep can
 affect fiemap result

On Fri, Apr 07, 2017 at 01:02:58PM +0800, Eryu Guan wrote:
> On Thu, Apr 06, 2017 at 11:28:01AM -0500, Eric Sandeen wrote:
> > On 4/6/17 11:26 AM, Theodore Ts'o wrote:
> > > On Wed, Apr 05, 2017 at 10:35:26AM +0800, Eryu Guan wrote:
> > >>
> > >> Test fails with ext3/2 when driving with ext4 driver, fiemap changed
> > >> after umount/mount cycle, then changed back to original result after
> > >> sleeping some time. An ext4 bug? (cc'ed linux-ext4 list.)
> > > 
> > > I haven't had time to look at this, but I'm not sure this test is a
> > > reasonable one on the face of it.
> > > 
> > > A file system may choose to optimize a file's extent tree for whatever
> > > reason it wants, whenever it wants, including on an unmount --- and
> > > that would not be an invalid thing to do.  So to have an xfstests that
> > > causes a test failure if a file system were to, say, do some cleanup
> > > at mount or unmount time, or when the file is next opened, to merge
> > > adjacent extents together (and hence change what is returned by
> > > FIEMAP) might be strange, or even weird --- but is this any of user
> > > space's business?  Or anything we want to enforce as wrong wrong wrong
> > > by xfstests?
> 
> So I was asking for a review from ext4 side instead of queuing it for
> next xfstests update :)

In general FIEMAP can return pretty much whatever it wants, which
usually means that it won't report extents larger than the underlying
block mapping extents, though as we've seen it can split a single
on-disk extent into multiple FIEMAP records for the purpose of reporting
sharedness.

For ext3 I'm wondering if it's the case that the first time we FIEMAP an
indirect map file we see a possibly-merged version of whatever's in the
particular leaf node we land in; then that information gets cached &
merged with other records in the extent status tree, such that
subsequent FIEMAP calls see longer extents than the first time around.

> > I had the same question.  If the exact behavior isn't defined anywhere,
> > I don't know what we can be testing, TBH.
> 
> Agreed, I was about to ask for the expected behavior today if there was
> no new review comments on this patch.

I think the expected behavior is that any behavior is expected. :(

--D

> 
> Thanks for the comments and review!
> 
> Eryu
> --
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