Bug 64616 - Not all cursors seem to work correctly.
Summary: Not all cursors seem to work correctly.
Status: NEW
Alias: None
Product: systemd
Classification: Unclassified
Component: general (show other bugs)
Version: unspecified
Hardware: Other All
: medium normal
Assignee: systemd-bugs
QA Contact: systemd-bugs
Depends on:
Reported: 2013-05-15 07:13 UTC by Marius Vollmer
Modified: 2013-05-16 07:21 UTC (History)
0 users

See Also:
i915 platform:
i915 features:

Ttry to avoid binary search when looking up by seqnum. (1.58 KB, patch)
2013-05-16 07:21 UTC, Marius Vollmer
Details | Splinter Review

Description Marius Vollmer 2013-05-15 07:13:25 UTC
I can see this curious behavior:

# ./journalctl -D /var/log/journal.cursor-bug -p3 -u rhsc-ws.service _BOOT_ID=a12b9c5d73a14fd49bcd51b99faa0c17 -o verbose | grep s=
Wed 2013-05-08 15:03:43 EDT [s=2d3aee2eda874445a91bcb0f92718eea;i=3d8;b=a12b9c5d73a14fd49bcd51b99faa0c17;m=261477778;t=4dc3997b0904f;x=a971f4f19117d9f]
Fri 2013-05-10 04:21:41 EDT [s=2d3aee2eda874445a91bcb0f92718eea;i=48b;b=a12b9c5d73a14fd49bcd51b99faa0c17;m=21a4e8dab1;t=4dc58db51f388;x=f89f9d5aaec9bed9]
# ./journalctl -D /var/log/journal.cursor-bug -c 's=2d3aee2eda874445a91bcb0f92718eea;i=48b;b=a12b9c5d73a14fd49bcd51b99faa0c17;m=21a4e8dab1;t=4dc58db51f388;x=f89f9d5aaec9bed9' -n1 -o verbose | grep s=
Mon 2013-05-13 02:49:19 EDT [s=c66de0241a874e3cac6e7a34baccf561;i=1;b=19efc07fb36244ae97f9e568897c4547;m=8cb56;t=4dc93ea840fa9;x=8e7e4637eff8a6d1]

That is, when feeding a certain cursor back into journalctl, it will fail to seek to that cursor and instead output something else.

Since the cursor has just been output by journalctl, I would expect it to be valid.

In any case, a error or warning message would be appropriate, I think.

I'll try to debug this right away.  If you are interested, I can upload 
/var/log/journal.cursor-bug somewhere.  It's about 23 MiB.
Comment 1 Marius Vollmer 2013-05-15 16:34:13 UTC
Right, it turned out that some of the files in /var/log/journal.cursor-bug are corrupted.  Some entry objects in them are zeroed-out.  I am quite brutal with this machine (a VM), so this is not that much of a surprise.

Unfortunately, the journal is not just shorter than expected, it really is corrupted and becomes inconsistent as shown in the bug description.

_Fortunately_, binary search doesn't really seem to be necessary for finding entries by seqnum.  A entry with seqnum S should always be at index S-1 in the entry array of its file, no?

This patch works for my scenario:

--- a/src/journal/journal-file.c
+++ b/src/journal/journal-file.c
@@ -1767,6 +1767,21 @@ int journal_file_move_to_entry_by_seqnum(
                 Object **ret,
                 uint64_t *offset) {
+        int r;
+        if (seqnum >= 1 && seqnum <= f->header->n_entries) {
+                Object *o;
+                r = generic_array_get(f,
+                                      le64toh(f->header->entry_array_offset),
+                                      seqnum-1,
+                                      &o, offset);
+                if (r >= 0 && le64toh(o->entry.seqnum) == seqnum) {
+                        if (ret)
+                                *ret = o;
+                        return 1;
+                }
+        }
         return generic_array_bisect(f,

(It is quite conservative, and falls back to the existing code if my assumption is wrong.)

I haven't yet looked at journal_file_move_to_entry_by_seqnum_for_data.  Something similar will probably be appropriate there as well.

Even without considering corruption, using generic_array_get with seqnums should be a small performance improvement, no?

In general, we might try to detect corruption during a binary search, issue a warning, and try to repair the damage.  For example, we might do a linear search for a non-corrupted entry and continue the bisecting from there.

I'll look at the _data case and then make a complete patch.
Comment 2 Marius Vollmer 2013-05-16 07:20:21 UTC
The _data case needs binary search, and seqnums are probably not guaranteed to strictly correspond to array indices.  Correct?

I am happy for now with the attached patch, but that doesn't do much about robustness against file corruption.
Comment 3 Marius Vollmer 2013-05-16 07:21:24 UTC
Created attachment 79382 [details] [review]
Ttry to avoid binary search when looking up by seqnum.

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