Re: [PATCH 2/2] f2fs: support revoking atomic written pages

[Jaegeuk Kim <jaegeuk@kernel.org>]

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?
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.

> > 
> > > 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?

> 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

Let me verify these scenarios first. :)

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@kernel.org>
> > > > 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@kernel.org>
> > > > ---
> > > >  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
> > >