On Fri, 2024-02-23 at 21:33 +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, 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 Hi, I can do it this way for sure as well. Another point still in my mind is the superblock: I would like to get rid of the build time switch and carry information about lazy commits enabled in the superblock. As there is J, B, D and R as mode already, a new mode L or such could be added. I will work on this and also take a look at stuff like dmsetup to check if something would be needed there. If there are further points for now on anyone's mind, please tell. Best, Simone
