On Mon 14-11-22 22:08:16, Amir Goldstein wrote: > On Mon, Nov 14, 2022 at 9:17 PM Jan Kara <jack@xxxxxxx> wrote: > > > My understanding is that > > > synchronize_srcu(&fsnotify_mark_srcu); > > > is needed as barrier between 3) and 4) > > > > > > In any case, even if CHECKPOINT_PENDING can work, > > > with or without FAN_MARK_SYNC, to me personally, understanding > > > the proof of correctness of alternating groups model is very easy, > > > while proving correctness for CHECKPOINT_PENDING model is > > > something that I was not yet able to accomplish. > > > > I agree the scheme with CHECKPOINT_PENDING isn't easy to reason about but I > > don't find your scheme with two groups simpler ;) Let me try to write down > > rationale for my scheme, I think I can even somewhat simplify it: > > > > Write operation consists of: > > generate PRE_WRITE on F > > modify data of F > > generate POST_WRITE on F > > > > Checkpoint consists of: > > clear ignore marks > > report files for which we received PRE_WRITE or POST_WRITE until this > > moment > > > > And the invariant we try to provide is: If file F was modified during > > checkpoint T, then we report F as modified during T or some later > > checkpoint. Where it is matter of quality of implementation that "some > > later checkpoint" isn't too much later than T but it depends on the > > frequency of checkpoints, the length of notification queue, etc. so it is > > difficult to give hard guarantees. > > > > And the argument why the scheme maintains the invariant is that if > > POST_WRITE is generated after "clear ignore marks" finishes, it will get > > delivered and thus F will get reported as modified in some checkpoint once > > the event is processed. If POST_WRITE gets generated before "clear ignore > > marks" finishes and F is among ignored inodes, it means F is already in > > modified set so it will get reported as part of checkpoint T. Also > > application will already see modified data when processing list of modified > > files in checkpoint T. > > > > Simple and we don't even need PRE_WRITE here. But maybe you wanted to > > provide stronger invariant? Like "you are not able to see modified data > > without seeing F as modified?" But what exactly would be a guarantee here? > > I feel I'm missing something here but I cannot quite grasp it at this > > moment... > > > > This is the very basic guarantee that the persistent change tracking snapshots > need to provide. If a crash happens after modification is done and before > modification is recorded, we won't detect the modification after reboot. Right, crash safety was the point I was missing ;) Thanks for reminding me. And now I also see why you use filesystem freezing - it is a way to make things crash safe as otherwise it is difficult to guard against a race generate PRE_WRITE for F PRE_WRITE ignored because file is already modified checkpoint happens -> F reported as modified contents of F fetched modify data transaction commit <crash> POST_WRITE never seen so change to F is never reported I just think filesystem freezing is too big hammer for widespread use of persistent change tracking. Can't we introduce some SRCU lock / unlock into file_start_write() / file_end_write() and then invoke synchronize_srcu() during checkpoint after removing ignore marks? It will be much cheaper as we don't have to flush all dirty data to disk and also writes can keep flowing while we wait for outstanding writes straddling checkpoint to complete. What do you think? The checkpoint would then do: start gathering changes for both T and T+1 clear ignore marks synchronize_srcu() stop gathering changes for T and report them And in this case we would not need POST_WRITE as an event. The technical problem I see is how to deal with AIO / io_uring because SRCU needs to be released in the same context as it is acquired - that would need to be consulted with Paul McKenney if we can make it work. And another problem I see is that it might not be great to have this system-wide as e.g. on networking filesystems or pipes writes can block for really long. Final question is how to expose this to userspace because this functionality would seem useful outside of filesystem notification space so probably do not need to tie it to that. Or we could simplify our life somewhat and acquire SRCU when generating PRE_WRITE and drop it when generating POST_WRITE. This would keep SRCU within fsnotify and would mitigate the problems coming from system-wide SRCU. OTOH it will create problems when PRE_WRITE gets generated and POST_WRITE would not for some reason. Just branstorming here, I've not really decided what's better... > Maybe "checkpoint" was a bad name to use for this handover between > two subsequent change tracking snapshots. I'm getting used to the terminology :) But to me it still seems more natural to look at the situation as a single stream of events where we fetch bulks of changes at certain moments, rather than looking at it as certain entities collecting events for different time intervals. > > > > > I am going to try to implement the PRE/POST_WRITE events and for > > > > > POC I may start with a single group because it may be easier or I may > > > > > implement both schemes, we'll see. > > > > > > > > That would be great. Thank you. Perhaps I'm missing something in my mental > > > > model which would make things impractical :) > > > > > > > > > > Me too. I won't know before I try. > > > > > > FYI, at the moment I am considering not allowing independent > > > subscription for PRE/POST_XXX events, but only subscribe to > > > XXX events and a group with class FAN_CLASS_VFS_FILTER > > > will get both PRE/POST_XXX and won't be able to subscribe > > > to XXX events that do not have PRE_XXX events. > > > > > > The rationale is that if a group subscribes to either PRE/POST_XXX > > > XXX() operation is not going to be on the fast path anyway. > > > > > > This will make it easier to support more PRE/POST_XXX events > > > without using up all the remaining 32bits namespace. > > > > > > Using the high 32bits of mask for PRE events and folding them > > > in the object interest mask with the low 32bit is another thing > > > that I was considering in case you would prefer to allow > > > independent subscription for PRE/POST_XXX events. > > > > So one question: Do you see anything else than POST_WRITE as being useful? > > For directory operations it seems pointless as they hold i_rwsem exclusively > > so I don't see useful distinction between PRE and POST event. For OPEN and > > CLOSE I don't see use either. ACCESS might be the only one where PRE and > > POST would both be useful for something. > > PRE_ACCESS is used to populate missing data and POST_ACCESS > is irrelevant for that. > > PRE_MODIFY is used for something completely different, it is used > for the persistent change tracking and this has to be crash safe, so > exclusive i_rwsem has nothing to do with it. > > PRE_MODIFY is called before i_rwsem and before sb_start_write() > so that HSM can record the change intent in the same filesystem > that the change is going to happen (this is a journaled record). > > I feel I may have not explained this correctly. > Does this make sense? Yes, makes sense. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
