Re: [PATCH RFC 00/18] xfs: atomic file updates

On Fri, May 01, 2020 at 09:46:07PM +0200, Jann Horn wrote:

On Wed, Apr 29, 2020 at 4:46 AM Darrick J. Wong [off-list ref] wrote:

quoted

This series creates a new log incompat feature and log intent items to
track high level progress of swapping ranges of two files and finish
interrupted work if the system goes down.  It then adds a new
FISWAPRANGE ioctl so that userspace can access the atomic extent
swapping feature.  With this feature, user programs will be able to
update files atomically by opening an O_TMPFILE, reflinking the source
file to it, making whatever updates they want to make, and then
atomically swap the changed bits back to the source file.  It even has
an optional ability to detect a changed source file and reject the
update.

The intent behind this new userspace functionality is to enable atomic
rewrites of arbitrary parts of individual files.  For years, application
programmers wanting to ensure the atomicity of a file update had to
write the changes to a new file in the same directory, fsync the new
file, rename the new file on top of the old filename, and then fsync the
directory.  People get it wrong all the time, and $fs hacks abound.

With atomic file updates, this is no longer necessary.  Programmers
create an O_TMPFILE, optionally FICLONE the file contents into the
temporary file, make whatever changes they want to the tempfile, and
FISWAPRANGE the contents from the tempfile into the regular file.

That also requires the *readers* to be atomic though, right? Since now
the updates are visible to readers instantly, instead of only on the
next open()? If you used this to update /etc/passwd while someone else
is in the middle of reading it with a sequence of read() calls, there
would be fireworks...

Right.  In XFS, we guarantee read atomicity by by grabbing i_rwsem and
the xfs mmap lock, break any layout leases, drain the directios, and
then flush+invalidate the page cache.  Once that preparation step is
done, we do the actual extent swap.

I guess maybe the new API could also be wired up to ext4's
EXT4_IOC_MOVE_EXT somehow, provided that the caller specifies
FILE_SWAP_RANGE_NONATOMIC?

Sort of.  ext4's MOVE_EXT also swaps the file contents doing the swap
one buffer_head at a time, so you'd have to turn that off since this API
assumes that the caller already set each file's contents beforehand.

Ted has theorized that so long as the extent map size is less than 1/4
of the journal then it would be possible to do atomic swaps in ext4
without adding all the logical log item bits that were a prerequisite
for the xfs implementation.

--D