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

[Chao Yu]

Hi Jaegeuk,

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> Sent: Thursday, December 31, 2015 3:41 AM
> To: Chao Yu
> Cc: linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 2/2] f2fs: support revoking atomic written pages
> 
> Hello,
> 
> On Wed, Dec 30, 2015 at 09:34:40AM +0800, Chao Yu wrote:
> > Hi Jaegeuk,
> >
> > > -----Original Message-----
> > > From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> > > Sent: Wednesday, December 30, 2015 8:05 AM
> > > To: Chao Yu
> > > Cc: linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > > Subject: Re: [PATCH 2/2] f2fs: support revoking atomic written pages
> > >
> > > 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;

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?

> 
> > 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.

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).

> 
> > 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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