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Re: [PATCH] xfstests 255: add a seek_data/seek_hole tester

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfstests 255: add a seek_data/seek_hole tester
From: Josef Bacik <josef@xxxxxxxxxx>
Date: Wed, 29 Jun 2011 09:19:41 -0400
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx
In-reply-to: <20110629065306.GC1026@dastard>
References: <1309275199-10801-1-git-send-email-josef@xxxxxxxxxx> <1309275199-10801-5-git-send-email-josef@xxxxxxxxxx> <20110629065306.GC1026@dastard>
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On 06/29/2011 02:53 AM, Dave Chinner wrote:
On Tue, Jun 28, 2011 at 11:33:19AM -0400, Josef Bacik wrote:
This is a test to make sure seek_data/seek_hole is acting like it does on
Solaris.  It will check to see if the fs supports finding a hole or not and will
adjust as necessary.

So I just looked at this with an eye to validating an XFS
implementation, and I came up with this list of stuff that the test
does not cover that I'd need to test in some way:

        - files with clean unwritten extents. Are they a hole or
          data? What's SEEK_DATA supposed to return on layout like
          hole-unwritten-data? i.e. needs to add fallocate to the

        - files with dirty unwritten extents (i.e. dirty in memory,
          not on disk). They are most definitely data, and most
          filesystems will need a separate lookup path to detect
          dirty unwritten ranges because the state is kept
          separately (page cache vs extent cache).  Plenty of scope
          for filesystem specific bugs here so needs a roubust test.

        - cold cache behaviour - all dirty data ranges the test
          creates are hot in cache and not even forced to disk, so
          it is not testing the no-page-cache-over-the-data-range
          case. i.e. it tests delalloc state tracking but not
          data-extent-already exists lookups during a seek.

        - assumes that allocation size is the block size and that
          holes follows block size alignment. We already know that
          ext4 does not follow that rule when doing small sparse
          writes close together in a file, and XFS is also known to
          fill holes when doing sparse writes past EOF.

        - only tests single block data extents ѕo doesn't cover
          corner cases like skipping over multiple fragmented data
          extents to the next hole.

Yeah I intentionally left out preallocated stuff because these are going to be implementation specific, so I was going to leave that for a later exercise when people actually start doing proper implementations.

Some more comments in line....

+    rm -f $tmp.*
+trap "_cleanup ; exit \$status" 0 1 2 3 15
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+# real QA test starts here
+_supported_fs generic
+_supported_os Linux

The log file is usually named $seq.full, and doesn't get placed in
the filesystem being tested. It gets saved in the xfstests directory
along side $seq.out.bad for analysis whenteh test fails...

I only want it to see if SEEK_HOLE fails so I can say it didn't run. I followed the same example as the fiemap test that Eric wrote.

+[ -x $here/src/seek-tester ] || _notrun "seek-tester not built"
+       rm -f $testfile
+       rm -f $logfile
+trap "_cleanup; exit \$status" 0 1 2 3 15
+echo "Silence is golden"
+$here/src/seek-tester -q $testfile 2>&1 | tee -a $logfile

Personally I'd prefer the test to be a bit noisy about what it is
running, especially when there are so many subtests the single
invocation is running. It makes no difference to the run time ofthe
test, or the output when something fails, but it at least allows you
to run the test manually and see what it is doing easily...

Right, the problem with this test is it will run differently depending on the implementation. I agree, I really like the noisy output tests, but unfortunately if I run this test on ext4 where it currently treats the entire file as data, and then run it on btrfs where it is smarter and actually recognizes holes, we end up with two different outputs that are both correct.

+if grep -q "SEEK_HOLE is not supported" $logfile; then
+       _notrun "SEEK_HOLE/SEEK_DATA not supported by this kernel"
+rm -f $logfile
+rm -f $testfile
+status=0 ; exit
diff --git a/255.out b/255.out
new file mode 100644
index 0000000..7eefb82
--- /dev/null
+++ b/255.out
@@ -0,0 +1,2 @@
+QA output created by 255
+Silence is golden
diff --git a/group b/group
index 1f86075..c045e70 100644
--- a/group
+++ b/group
@@ -368,3 +368,4 @@ deprecated
  252 auto quick prealloc
  253 auto quick
  254 auto quick
+255 auto quick

I'd suggest that rw and prealloc (once unwritten extent
testing is added) groups should also be defined for this test.

Otherwise, the test code looks ok if a bit over-engineered....

+struct testrec {
+       int     test_num;
+       int     (*test_func)(int fd, int testnum);
+       char    *test_desc;
+struct testrec seek_tests[] = {
+       {  1, test01, "Test basic support" },
+       {  2, test02, "Test an empty file" },
+       {  3, test03, "Test a full file" },
+       {  4, test04, "Test file hole at beg, data at end" },
+       {  5, test05, "Test file data at beg, hole at end" },
+       {  6, test06, "Test file hole data hole data" },

So, to take from the hole punch test matrix, it covers a bunch more
file state transitions and cases that are just as relevant to
SEEK_HOLE/SEEK_DATA. Those cases are:

#       1. into a hole
#       2. into allocated space
#       3. into unwritten space
#       4. hole ->  data
#       5. hole ->  unwritten
#       6. data ->  hole
#       7. data ->  unwritten
#       8. unwritten ->  hole
#       9. unwritten ->  data
#       10. hole ->  data ->  hole
#       11. data ->  hole ->  data
#       12. unwritten ->  data ->  unwritten
#       13. data ->  unwritten ->  data
#       14. data ->  hole @ EOF
#       15. data ->  hole @ 0
#       16. data ->  cache cold ->hole
#       17. data ->  hole in single block file

I thikn we also need to cover most of these same cases, right?  And
SEEK_HOLE/SEEK data also need to explicitly separate the unwritten
tests into "clean unwritten" and "dirty unwritten" and cover the
transitions between regions of those states as well, right?

Yeah you are right, but again doing preallocated stuff is tricky, but I can expand it now if that's what we want. Thanks,


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