RE: [f2fs-dev] [PATCH 2/2] f2fs: support revoking atomic written pages

[Chao Yu]

Hi Jaegeuk,

Any progress on this patch?

Thanks,

> -----Original Message-----
> From: Chao Yu [mailto:chao@xxxxxxxxxx]
> Sent: Friday, January 01, 2016 8:14 PM
> To: Jaegeuk Kim
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [f2fs-dev] [PATCH 2/2] f2fs: support revoking atomic written pages
> 
> Hi Jaegeuk,
> 
> On 1/1/16 11:50 AM, Jaegeuk Kim wrote:
> > Hi Chao,
> >
> > ...
> >
> >>>>> On Tue, Dec 29, 2015 at 11:12:36AM +0800, Chao Yu wrote:
> >>>>>> f2fs support atomic write with following semantics:
> >>>>>> 1. open db file
> >>>>>> 2. ioctl start atomic write
> >>>>>> 3. (write db file) * n
> >>>>>> 4. ioctl commit atomic write
> >>>>>> 5. close db file
> >>>>>>
> >>>>>> With this flow we can avoid file becoming corrupted when abnormal power
> >>>>>> cut, because we hold data of transaction in referenced pages linked in
> >>>>>> inmem_pages list of inode, but without setting them dirty, so these data
> >>>>>> won't be persisted unless we commit them in step 4.
> >>>>>>
> >>>>>> But we should still hold journal db file in memory by using volatile write,
> >>>>>> because our semantics of 'atomic write support' is not full, in step 4, we
> >>>>>> could be fail to submit all dirty data of transaction, once partial dirty
> >>>>>> data was committed in storage, db file should be corrupted, in this case,
> >>>>>> we should use journal db to recover the original data in db file.
> >>>>>
> >>>>> Originally, IOC_ABORT_VOLATILE_WRITE was supposed to handle commit failures,
> >>>>> since database should get its error literally.
> >>>>>
> >>>>> So, the only thing that we need to do is keeping journal data for further db
> >>>>> recovery.
> >>>>
> >>>> IMO, if we really support *atomic* interface, we don't need any journal data
> >>>> kept by user, because f2fs already have it in its storage since we always
> >>>> trigger OPU for pages written in atomic-write opened file, f2fs can easily try
> >>>> to revoke (replace old to new in metadata) when any failure exist in atomic
> >>>> write process.
> >>>
> >>> Yeah, so current design does not fully support atomic writes. IOWs, volatile
> >>> writes for journal files should be used together to minimize sqlite change as
> >>> much as possible.
> >>>
> >>>> But in current design, we still hold journal data in memory for recovering for
> >>>> *rare* failure case. I think there are several issues:
> >>>> a) most of time, we are in concurrent scenario, so if large number of journal
> >>>> db files were opened simultaneously, we are under big memory pressure.
> >>>
> >>> In current android, I've seen that this is not a big concern. Even there is
> >>> memory pressure, f2fs flushes volatile pages.
> >>
> >> When I change to redirty all volatile pages in ->writepage, android seems go
> >> into an infinite loop when doing recovery flow of f2fs data partition in startup.
> >>
> >> if (f2fs_is_volatile_file(inode))
> >> 	goto redirty_out;
> >
> > Where did you put this? It doesn't flush at all? Why?
> 
> Original place in ->writepage, just remove two other conditions.
> 
> To avoid potential random writebacking of dirty page in journal which
> cause unpredicted corrupting in journal.
> 
> > Practically, the peak amount of journal writes depend on how many transactions
> > are processing concurrently.
> > I mean, in-memory pages are dropped at the end of every transaction.
> > You can check the number of pages through f2fs_stat on your phone.
> >
> >> I didn't dig details, but I think there may be a little risk for this design.
> >>
> >>>
> >>>> b) If we are out of memory, reclaimer tries to write page of journal db into
> >>>> disk, it will destroy db file.
> >>>
> >>> I don't understand. Could you elaborate why journal writes can corrupt db?
> >>
> >> Normally, we keep pages of journal in memory, but partial page in journal
> >> will be write out to device by reclaimer when out of memory. So this journal
> >> may have valid data in its log head, but with corrupted data, then after
> >> abnormal powe-cut, recovery with this journal before a transaction will
> >> destroy db. Right?
> >
> > Just think about sqlite without this feature.
> > Broken journal is pretty normal case for sqlite.
> 
> Maybe, if it is caused by bug or design issue of software, no matter db system
> or filesystem, we should try our best to fix it to avoid generating broken journals.
> 
> >
> >>>
> >>>> c) Though, we have journal db file, we will face failure of recovering db file
> >>>> from journal db due to ENOMEM or EIO, then db file will be corrupted.
> >>>
> >>> Do you mean the failure of recovering db with a complete journal?
> >>> Why do we have to handle that? That's a database stuff, IMO.
> >>
> >> Yes, just list for indicating we will face the same issue which is hard to
> >> handle both in original design and new design, so the inner revoking failure
> >> issue would not be a weak point or flaw of new design.
> >>
> >>>
> >>>> d) Recovery flow will make data page dirty, triggering both data stream and
> >>>> metadata stream, there should be more IOs than in inner revoking in
> >>>> atomic-interface.
> >>>
> >>> Well, do you mean there is no need to recover db after revoking?
> >>
> >> Yes, revoking make the same effect like the recovery of sqlite, so after
> >> revoking, recovery is no need.
> >
> > Logically, it doesn't make sense. If there is a valid journal file, it should
> > redo the previous transaction. No?
> 
> As we know, in sqlite, before we commit a transaction, we will use journal to
> record original data of pages which will be updated in following transaction, so
> in following if a) abnormal power-cut, b) commit error, c) redo command was
> triggered by user, we will recover db with journal.
> 
> Ideally, if we support atomic write interface, in there should always return two
> status in atomic write interface: success or fail. If success, transaction was
> committed, otherwise, it looks like nothing happened, user will be told
> transaction was failed. Then, journals in sqlite could no longer be used,
> eventually no journal, no recovery.
> 
> The only thing we should concern is inner failure (e.g. ENOMEM, ENOSPC) of
> revoking in commit interface since it could destroy db file permanently w/o
> journal. IMO, some optimization could be done for these cases:
> 1. ENOMEM: enable retrying or mark accessed flag in page in advance.
> 2. ENOSPC: preallocate blocks for node blocks and data blocks.
> 
> These optimizations couldn't guarantee no failure in revoking operation
> completely, luckily, those are not common cases, and they also happen in sqlite
> w/o atomic feature.
> 
> One more possible proposal is: if we support reflink feature like ocfs2/xfs, I
> guess we can optimize DB like:
> 1. reflink db to db.ref
> 2. do transaction in db.ref
>    - failed, rm db.ref
>    - power-cut rm db.ref
> 3. rename db.ref to db
> 
> >
> >> One more case is that user can send a command to abort current transaction,
> >> it should be happened before atomic_commit operation, which could easily
> >> handle with abort_commit ioctl.
> >>
> >>>
> >>>> e) Moreover, there should be a hole between 1) commit fail and 2) abort write &
> >>>> recover, checkpoint will persist the corrupt data in db file, following abnormal
> >>>> power-cut will leave that data in disk.
> >>>
> >>> Yes, in that case, database should recover corrupted db with its journal file.
> >>
> >> Journal could be corrupted as I descripted in b).
> >
> > Okay, so what I'm thinking is like this.
> > It seems there are two corruption cases after journal writes.
> >
> > 1. power cut during atomic writes
> >  - broken journal file and clean db file -> give up
> >  - luckily, valid journal file and clean db file -> recover db
> >
> > 2. error during atomic writes
> >  a. power-cut before abort completion
> >   - broken journal file and broken db file -> revoking is needed!
> >
> >  b. after abort
> >   - valid journal file and broken db file -> recover db (likewise plain sqlite)
> >
> > Indeed, in the 2.a. case, we need revoking; I guess that's what you mentioned.
> > But, I think, even if revoking is done, we should notify an error to abort and
> > recover db by 2.b.
> >
> > Something like this after successful revoking.
> >
> > 1. power cut during atomic writes
> >  - broken journal file and clean db file -> give up
> >  - luckily, valid journal file and clean db file -> recover db
> >
> > 2. error during atomic writes w/ revoking
> >  a. power-cut before abort completion
> >   - broken journal file and clean db file -> give up
> >   - luckily, valid journal file and clean db file -> recover db
> >
> >  b. after abort
> >   - valid journal file and clean db file -> recover db
> 
> That's right.
> 
> >
> > Let me verify these scenarios first. :)
> 
> OK. :)
> 
> Thanks,
> 
> >
> > Thanks,
> >
> >>>
> >>>> With revoking supported design, we can not solve all above issues, we will still
> >>>> face the same issue like c), but it will be a big improve if we can apply this
> >>>> in our interface, since it provide a way to fix the issue a) b) d). And also for
> >>>> e) case, we try to rescue data in first time that our revoking operation would be
> >>>> protected by f2fs_lock_op to avoid checkpoint + power-cut.
> >>>>
> >>>> If you don't want to have a big change in this interface or recovery flow, how
> >>>> about keep them both, and add a mount option to control inner recovery flow?
> >>>
> >>> Hmm, okay. I believe the current design is fine for sqlite in android.
> >>
> >> I believe new design will enhance in memory usage and error handling of sqlite
> >> in android, and hope this can be applied. But, I can understand that if you
> >> were considerring about risk control and backward compatibility, since this
> >> change affects all atomic related ioctls.
> >>
> >>> For other databases, I can understand that they can use atomic_write without
> >>> journal control, which is a sort of stand-alone atomic_write.
> >>>
> >>> It'd better to add a new ioctl for that, but before adding it, can we find
> >>> any usecase for this feature? (e.g., postgresql, mysql, mariadb, couchdb?)
> >>
> >> You mean investigating or we can only start when there is a clear commercial
> >> demand ?
> >>
> >>> Then, I expect that we can define a more appropriate and powerful ioctl.
> >>
> >> Agreed :)
> >>
> >> Thanks,
> >>
> >>>
> >>> Thanks,
> >>>
> >>>>
> >>>> How do you think? :)
> >>>>
> >>>> Thanks,
> >>>>
> >>>>> But, unfortunately, it seems that something is missing in the
> >>>>> current implementation.
> >>>>>
> >>>>> So simply how about this?
> >>>>>
> >>>>> A possible flow would be:
> >>>>> 1. write journal data to volatile space
> >>>>> 2. write db data to atomic space
> >>>>> 3. in the error case, call ioc_abort_volatile_writes for both journal and db
> >>>>>  - flush/fsync journal data to disk
> >>>>>  - drop atomic data, and will be recovered by database with journal
> >>>>>
> >>>>> From cb33fc8bc30981c370ec70fe68871130109793ec Mon Sep 17 00:00:00 2001
> >>>>> From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> >>>>> Date: Tue, 29 Dec 2015 15:46:33 -0800
> >>>>> Subject: [PATCH] f2fs: fix f2fs_ioc_abort_volatile_write
> >>>>>
> >>>>> There are two rules to handle aborting volatile or atomic writes.
> >>>>>
> >>>>> 1. drop atomic writes
> >>>>>  - we don't need to keep any stale db data.
> >>>>>
> >>>>> 2. write journal data
> >>>>>  - we should keep the journal data with fsync for db recovery.
> >>>>>
> >>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> >>>>> ---
> >>>>>  fs/f2fs/file.c | 13 ++++++++++---
> >>>>>  1 file changed, 10 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> >>>>> index 91f576a..d16438a 100644
> >>>>> --- a/fs/f2fs/file.c
> >>>>> +++ b/fs/f2fs/file.c
> >>>>> @@ -1433,9 +1433,16 @@ static int f2fs_ioc_abort_volatile_write(struct file *filp)
> >>>>>  	if (ret)
> >>>>>  		return ret;
> >>>>>
> >>>>> -	clear_inode_flag(F2FS_I(inode), FI_ATOMIC_FILE);
> >>>>> -	clear_inode_flag(F2FS_I(inode), FI_VOLATILE_FILE);
> >>>>> -	commit_inmem_pages(inode, true);
> >>>>> +	if (f2fs_is_atomic_file(inode)) {
> >>>>> +		clear_inode_flag(F2FS_I(inode), FI_ATOMIC_FILE);
> >>>>> +		commit_inmem_pages(inode, true);
> >>>>> +	}
> >>>>> +	if (f2fs_is_volatile_file(inode)) {
> >>>>> +		clear_inode_flag(F2FS_I(inode), FI_VOLATILE_FILE);
> >>>>> +		ret = commit_inmem_pages(inode, false);
> >>>>> +		if (!ret)
> >>>>> +			ret = f2fs_sync_file(filp, 0, LLONG_MAX, 0);
> >>>>> +	}
> >>>>>
> >>>>>  	mnt_drop_write_file(filp);
> >>>>>  	return ret;
> >>>>> --
> >>>>> 2.6.3
> >>>>
> >
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