Re: [PATCH] namei: implemented RENAME_NEWER flag for renameat2() conditional replace

On Wed, Jun 29, 2022 at 12:56 AM James Yonan [off-list ref] wrote:

On 6/28/22 03:46, Amir Goldstein wrote:

quoted

[+linux-api]

On Tue, Jun 28, 2022 at 1:58 AM James Yonan [off-list ref] wrote:

quoted

RENAME_NEWER is a new userspace-visible flag for renameat2(), and
stands alongside existing flags such as RENAME_NOREPLACE,
RENAME_EXCHANGE, and RENAME_WHITEOUT.

RENAME_NEWER is a conditional variation on RENAME_NOREPLACE, and
indicates that if the target of the rename exists, the rename will
only succeed if the source file is newer than the target (i.e. source
mtime > target mtime).  Otherwise, the rename will fail with -EEXIST
instead of replacing the target.  When the target doesn't exist,
RENAME_NEWER does a plain rename like RENAME_NOREPLACE.

RENAME_NEWER is very useful in distributed systems that mirror a
directory structure, or use a directory as a key/value store, and need
to guarantee that files will only be overwritten by newer files, and
that all updates are atomic.

This feature sounds very cool.
For adding a new API it is always useful if you bring forward a userland
tool (rsync?) that intend to use it, preferably with a POC patch.
A concrete prospective user is always better than a hypothetical one.
Some people hold the opinion that only new APIs with real prospective
users should be merged.

Not sure that rsync would be the canonical user, though it might be a
reasonable POC.  The problem that we are solving is essentially
near-real-time directory mirroring or replication of one source
directory to many target directories on follower nodes.  Many writers,
many readers, filesystem-based, strong guarantees of eventual
convergence, infinitely scalable.  You have a source directory that
could be an AWS S3 bucket.  You have potentially thousands of follower
nodes that want to replicate the source directory on a local
filesystem.  You have messages flying around the network (Kafka, AWS
SQS, etc.) representing file updates.  These messages might be reordered
or duplicated but each contains the file content and a unique
nanosecond-scale timestamp.  Because the file update throughput can be
in the thousands of files per second, you might have multiple threads on
each node, receiving updates, and moving them into the target
directory.  The only way to guarantee that the state of the mirror
target directory on all nodes converges to the state of the source
directory is to have the last-step move operation be atomic and
conditional to the modification time (so that an earlier version of the
file doesn't overwrite a later version).

So I understand that there needs to be a strong case to extend the Linux
API, and I think the argument is that conditional atomic file operations
enable entirely new classes of applications.  The fact that this can be

Sure it does, but is the condition that you propose going to serve all
the prospect applications or just your application?
You may add to your arguments in favor of mtime that 'mv --update'
and 'rsync --update' could use the new flag.

facilitated by essentially 5 lines of kernel code is remarkable.

quoted

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While this patch may appear large at first glance, most of the changes
deal with renameat2() flags validation, and the core logic is only
5 lines in the do_renameat2() function in fs/namei.c:

         if ((flags & RENAME_NEWER)
             && d_is_positive(new_dentry)
             && timespec64_compare(&d_backing_inode(old_dentry)->i_mtime,
                                   &d_backing_inode(new_dentry)->i_mtime) <= 0)
                 goto exit5;

I have a few questions:
- Why mtime?
- Why not ctime?
- Shouldn't a feature like that protect from overwriting metadata changes
   to the destination file?

In any case, I would be much more comfortable with comparing ctime
because when it comes to user settable times, what if rsync *wants*
to update the destination file's mtime to an earlier time that was set in
the rsync source?

If comparing ctime does not fit your use case and you can convince
the community that comparing mtime is a justified use case, we would
need to use a flag name to reflect that like RENAME_NEWER_MTIME
so we don't block future use case of RENAME_NEWER_CTIME.
I hope that we can agree that ctime is enough and that mtime will not
make sense so we can settle with RENAME_NEWER that means ctime.

So I actually think that mtime is the better timestamp to use because
ctime is modified by the rename operation itself, while mtime measures
the last modification time of the file content, which is what we care about.

That is fine. I am saying there are other use cases that replicate not
only data, but metadata too. Even Cloud files have metadata attached,
so one day, someone else may want to compare ctime, which makes
the work NEWER ambiguous and you need to disambiguate it with
something like RENAME_EXCHANGE_NEWER_MTIME.

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It's pretty cool in a way that a new atomic file operation can even be
implemented in just 5 lines of code, and it's thanks to the existing
locking infrastructure around file rename/move that these operations
become almost trivial.  Unfortunately, every fs must approve a new
renameat2() flag, so it bloats the patch a bit.

So one question to ask is could this functionality be implemented
in userspace without adding a new renameat2() flag?  I think you
could attempt it with iterative RENAME_EXCHANGE, but it's hackish,
inefficient, and not atomic, because races could cause temporary
mtime backtracks.  How about using file locking?  Probably not,
because the problem we want to solve is maintaining file/directory
atomicity for readers by creating files out-of-directory, setting
their mtime, and atomically moving them into place.  The strategy
to lock such an operation really requires more complex locking methods
than are generally exposed to userspace.  And if you are using inotify
on the directory to notify readers of changes, it certainly makes
sense to reduce unnecessary churn by preventing a move operation
based on the mtime check.

While some people might question the utility of adding features to
filesystems to make them more like databases, there is real value
in the performance, atomicity, consistent VFS interface, multi-thread
safety, and async-notify capabilities of modern filesystems that
starts to blur the line, and actually make filesystem-based key-value
stores a win for many applications.

Like RENAME_NOREPLACE, the RENAME_NEWER implementation lives in
the VFS, however the individual fs implementations do strict flags
checking and will return -EINVAL for any flag they don't recognize.
For this reason, my general approach with flags is to accept
RENAME_NEWER wherever RENAME_NOREPLACE is also accepted, since
RENAME_NEWER is simply a conditional variant of RENAME_NOREPLACE.

You are not taking into account that mtime may be cached attribute that is not
uptodate in network filesystems (fuse too) so behavior can be a bit
unpredictable,
unless filesystem code compares also the cache coherency of the attributes
on both files and even then, without extending the network protocol this is
questionable behavior for client side.
So I think your filter of which filesystems to enable is way too wide.

So I'm new to the filesystem code, but my reading of do_renameat2() in
fs/namei.c seems to indicate that if d_is_positive(dentry) is true, it's
safe to access the inode struct via d_backing_inode(dentry) and
dereference mtime.  But what you're saying is that the mtime value
cached in the inode struct might not be up-to-date for network filesystems?

In the client's cache A may be newer than B, while on the server B is actually
newer, but that doesn't matter because...

quoted

How many of them did you test, I'll take a wild guess that not all of them.

Please do not enable RENAME_NEWER is any filesystem that you did
not test or that was not tested by some other fs developer using the
tests that you write.

So I'm mostly interested in implementing this on local filesystems,

... so don't add the functionality to filesystems that you don't need to add to
and you do not test.

LTP all_filesystems will give you coverage for all the local fs that you should
care about. Other fs could add support for the new flag themselves if the
developers care and test it - that's not your job when adding a new API.

because the application layer has already done the heavy lifting on the
networking side so that the filesystem layer can be local, fast, and
atomic.  So yes, I haven't tested this yet on networked filesystems.
But I'm thinking that because all functionality is implemented at the
VFS layer, it should be portable to any fs that also supports
RENAME_NOREPLACE, with the caveat that it depends on the ability of the
VFS to get a current and accurate mtime attribute inside the critical
section between lock_rename() and unlock_rename().

The implementation is generic. You just implement the logic in the vfs and
enable it for a few tested filesystems and whoever wants to join the party
is welcome to test their own filesystems and opt-in to the new flag whether
they like. Nothing wrong with that.

w.r.t stability of i_mtime, if I am not mistaken i_mtime itself is
stable with inode
lock held (i.e. after lock_two_nondirectories()), however, as Dave pointed out,
the file's data can be modified in page cache, so as long as the file is open
for write or mmaped writable, the check of mtime is not atomic.

Neil's suggestion to deny the operation on open files makes sense.
You can use a variant of deny_write_access() that takes inode
which implies the error  ETXTBSY for an attempt to exchange newer
with a file that is open for write.

Thanks,
Amir.