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 10:48:02 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Eryu Guan <eguan@...hat.com>
Cc:     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 4/7/17 10:42 AM, Darrick J. Wong wrote:
> 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. :(

I suppose that if any particular filesystem wants to enforce stricter
rules for its own fiemap interface, that could be done in a
filesystem-specific test...

-Eric