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

[Chao Yu]

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.

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.
b) If we are out of memory, reclaimer tries to write page of journal db into
disk, it will destroy db file.
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.
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.
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.

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?

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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