On Fri, 23 Feb 2024, Weiß, Simone wrote: > 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 I propose to use the commit ID 0 when writing the journal for the first time, then 1 when the journal rolls over, 2 when it rolls over again, 3 when it rolls over again, 4 on another roll over and so on up to 0x7fffffffffffffff (which will be never reached in practice). And use the top bit as an end-of-section marker. As the commit IDs will never roll over, it won't happen that an old transaction would be mixed into a new transaction on partial journal write. Mikulas
