On Tue, 2024-02-20 at 19:52 +0100, Mikulas Patocka wrote: > CAUTION: This email originated from outside of the Elektrobit organization. Do > not click links or open attachments unless you recognize the sender and know > the content is safe. > > > On Fri, 9 Feb 2024, Simone Weiß wrote: > > > Extend the dm-integrity driver to omit writing unused journal data sectors. > > Instead of filling up the whole journal section, mark the last used > > sector with a special commit ID. The commit ID still uses the same base > > value, > > but section number and sector number are inverted. At replay when commit IDs > > are analyzed this special commit ID is detected as end of valid data for > > this > > section. The main goal is to prolong the live times of e.g. eMMCs by > > avoiding > > to write the whole journal data sectors. > > > > The change is right now to be seen as experimental and gets applied if > > CONFIG_DMINT_LAZY_COMMIT is set to y. Note please that this is NOT > > planned for a final version of the changes. I would make it configurable > > via flags passed e.g. via dmsetup and stored in the superblock. > > > > Architectural Limitations: > > - A dm-integrity partition, that was previously used with lazy commit, > > can't be replayed with a dm-integrity driver not using lazy commit. > > - A dm-integrity driver that uses lazy commit is expected > > to be able to cope with a partition that was created and used without > > lazy commit. > > - With dm-integrity lazy commit, a partially written journal (e.g. due to a > > power cut) can cause a tag mismatch during replay if the journal entry > > marking > > the end of the journal section is missing. Due to lazy commit, older > > journal > > entries are not erased and might be processed if they have the same commit > > ID > > as adjacent newer journal entries. > > Hi > > I was thinking about it and I think that this problem is a showstopper. > > Suppose that a journal section contains these commit IDs: > > 2 2 2 2(EOF) 3 3 3 3 > > The IDs "3" are left over from previous iterations. The IDs "2" contain > the current data. And now, the journal rolls over and we attempt to write > all 8 pages with the ID "3". However, a power failure happens and we only > write 4 pages with the ID "3". So, the journal will look like: > > 3(new) 3(new) 3(new) 3(new) 3(old) 3(old) 3(old) 3(old) > > After a reboot, the journal-replay logic will falsely believe that the > whole journal section is consistent and it will attempt to replay it. > > This could be fixed by having always increasing commit IDs - the commit > IDs have 8 bytes, so we can assume that they never roll-over and it would > prevent us from mixing old IDs into the current transaction. Hi Thanks for the review of the concept. I was out this week and could only think about it now. I understood it right, that the proposal is to add an extra value to the commit ID, that is e.g. incremented when integrity_commit is executed? If so, I tried this quickly and looks good on first glance. Will check and test further next. Simone > > Mikulas > > > If dm-integrity detects bad sections while > > replaying the journal, keep track about those sections and try to at least > > replay older, good sections. > > This is based on the assumption that most likely the newest > > section(s) will be damaged, which might have been only partially written > > due to a sudden reset. Previously, the whole journal would be cleared in > > such a case. > > > > Signed-off-by: Simone Weiß <simone.weiss@xxxxxxxxxxxxxx> > > Signed-off-by: Kai Tomerius <kai.tomerius@xxxxxxxxxxxxxx>
