Re: [RFC PATCH v3 0/2] fs: add AT_REPLACE flag for linkat()

[Omar Sandoval <osandov@osandov.com>]

On Mon, Apr 23, 2018 at 11:19:40PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> Hi, Al,
> 
> This is a respin of my linkat() AT_REPLACE series, previously posted
> here:
> 
> https://patchwork.kernel.org/patch/9636735/
> https://patchwork.kernel.org/patch/9636733/
> 
> There are no changes since v2, only a rebase onto v4.17-rc2 and some
> minor additions to the commit messages.
> 
> The goal is to allow for updating a file atomically in-place with an
> O_TMPFILE like so:
> 
> - open temporary file with O_TMPFILE
> - write temporary file contents
> - fsync temporary file
> - atomically replace permanent location with the temporary file
> - fsync parent directory
> 
> Patch 1 implements the VFS support for this flag. The implementation
> resembles sys_renameat2(), and I took care to preserve all of the
> original error cases and make the new error cases consistent with
> rename.
> 
> Previously, we discussed extending renameat2() instead of linkat() for
> this, but this makes a mess of rename and also forces us to special-case
> the parent directory for O_TMPFILEs in implementations of
> i_op->rename(), so I still think linkat() is a better fit.
> 
> Patch 2 adds support for AT_REPLACE to Btrfs. That's the codebase I'm
> most familiar with so that's where I started, but it should be
> straightforward to implement for other filesystems. v1 of this series
> had some incorrect dentry fiddling, so now I just unhash the replaced
> dentry for simplicity.
> 
> Please take a look.
> 
> Thanks!

Ping, Al, can you take a look?