Making linkat() able to overwrite the target

Making linkat() able to overwrite the target

[David Howells]

With my rewrite of fscache and cachefiles:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter

when a file gets invalidated by the server - and, under some circumstances,
modified locally - I have the cache create a temporary file with vfs_tmpfile()
that I'd like to just link into place over the old one - but I can't because
vfs_link() doesn't allow you to do that.  Instead I have to either unlink the
old one and then link the new one in or create it elsewhere and rename across.

Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE, that
causes the target to be replaced and not give EEXIST?  Or make it so that
rename() can take a tmpfile as the source and replace the target with that.  I
presume that, either way, this would require journal changes on ext4, xfs and
btrfs.

Thanks,
David

Re: Making linkat() able to overwrite the target

[Al Viro]

On Tue, Jan 14, 2020 at 04:34:25PM +0000, David Howells wrote:
> With my rewrite of fscache and cachefiles:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter
> 
> when a file gets invalidated by the server - and, under some circumstances,
> modified locally - I have the cache create a temporary file with vfs_tmpfile()
> that I'd like to just link into place over the old one - but I can't because
> vfs_link() doesn't allow you to do that.  Instead I have to either unlink the
> old one and then link the new one in or create it elsewhere and rename across.
> 
> Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE, that
> causes the target to be replaced and not give EEXIST?  Or make it so that
> rename() can take a tmpfile as the source and replace the target with that.  I
> presume that, either way, this would require journal changes on ext4, xfs and
> btrfs.

Umm...  I don't like the idea of linkat() doing that - you suddenly get new
fun cases to think about (what should happen when the target is a mountpoint,
for starters?) _and_ you would have to add a magical flag to vfs_link() so
that it would know which tests to do.  As for rename...  How would that
work?  AT_EMPTY_PATH for source?  What happens if two threads do that
at the same time?  Should that case be always "create a new link, even
if you've got it by plain lookup somewhere"?  Worse, suppose you do that
to given tmpfile; what should happen to /proc/self/fd/* link to it?  Should
it point to new location, or...?

Re: Making linkat() able to overwrite the target

[David Howells]

Al Viro <viro@zeniv.linux.org.uk> wrote:

> > Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE,
> > that causes the target to be replaced and not give EEXIST?  Or make it so
> > that rename() can take a tmpfile as the source and replace the target with
> > that.  I presume that, either way, this would require journal changes on
> > ext4, xfs and btrfs.
>
> Umm...  I don't like the idea of linkat() doing that - you suddenly get new
> fun cases to think about (what should happen when the target is a mountpoint,
> for starters?

Don't allow it onto directories, S_AUTOMOUNT-marked inodes or anything that's
got something mounted on it.

> ) _and_ you would have to add a magical flag to vfs_link() so
> that it would know which tests to do.

Yes, I suggested AT_LINK_REPLACE as said magical flag.

> As for rename...

Yeah - with further thought, rename() doesn't really work as an interface,
particularly if a link has already been made.

Do you have an alternative suggestion?  There are two things I want to avoid:

 (1) Doing unlink-link or unlink-create as that leaves a window where the
     cache file is absent.

 (2) Creating replacement files in a temporary directory and renaming from
     there over the top of the target file as the temp dir would then be a
     bottleneck that spends a lot of time locked for creations and renames.

David

Re: Making linkat() able to overwrite the target

[Miklos Szeredi]

On Tue, Jan 14, 2020 at 7:06 PM David Howells <dhowells@redhat.com> wrote:
>
> Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> > > Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE,
> > > that causes the target to be replaced and not give EEXIST?  Or make it so
> > > that rename() can take a tmpfile as the source and replace the target with
> > > that.  I presume that, either way, this would require journal changes on
> > > ext4, xfs and btrfs.
> >
> > Umm...  I don't like the idea of linkat() doing that - you suddenly get new
> > fun cases to think about (what should happen when the target is a mountpoint,
> > for starters?
>
> Don't allow it onto directories, S_AUTOMOUNT-marked inodes or anything that's
> got something mounted on it.
>
> > ) _and_ you would have to add a magical flag to vfs_link() so
> > that it would know which tests to do.
>
> Yes, I suggested AT_LINK_REPLACE as said magical flag.
>
> > As for rename...
>
> Yeah - with further thought, rename() doesn't really work as an interface,
> particularly if a link has already been made.
>
> Do you have an alternative suggestion?  There are two things I want to avoid:
>
>  (1) Doing unlink-link or unlink-create as that leaves a window where the
>      cache file is absent.
>
>  (2) Creating replacement files in a temporary directory and renaming from
>      there over the top of the target file as the temp dir would then be a
>      bottleneck that spends a lot of time locked for creations and renames.

Create multiple sub-temp-dirs and use them alternatively.

I think there was a report for overlayfs with the same bottleneck
(copy up uses a temp dir, but now only for non-regular).  Hasn't
gotten around to implementing this idea yet.

Thanks,
Miklos

Re: Making linkat() able to overwrite the target

[Colin Walters]

On Tue, Jan 14, 2020, at 1:06 PM, David Howells wrote:

> Yes, I suggested AT_LINK_REPLACE as said magical flag.

This came up before right?

https://lore.kernel.org/linux-fsdevel/cover.1524549513.git.osandov@fb.com/

Re: Making linkat() able to overwrite the target

[Amir Goldstein]

On Fri, Jan 17, 2020 at 5:52 AM Colin Walters <walters@verbum.org> wrote:
>
> On Tue, Jan 14, 2020, at 1:06 PM, David Howells wrote:
>
> > Yes, I suggested AT_LINK_REPLACE as said magical flag.
>
> This came up before right?
>
> https://lore.kernel.org/linux-fsdevel/cover.1524549513.git.osandov@fb.com/

David,

This sounds like a good topic to be discussed at LSF/MM (hint hint)

Thanks,
Amir.

Re: Making linkat() able to overwrite the target

[David Howells]

Hi Omar,

Do you still have your AT_REPLACE patches?  You said that you'd post a v4
series, though I don't see it.  I could make use of such a feature in
cachefiles inside the kernel.  For my original question, see:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter

And do you have ext4 support for it?

Colin Walters <walters@verbum.org> wrote:

> On Tue, Jan 14, 2020, at 1:06 PM, David Howells wrote:
> 
> > Yes, I suggested AT_LINK_REPLACE as said magical flag.
> 
> This came up before right?
> 
> https://lore.kernel.org/linux-fsdevel/cover.1524549513.git.osandov@fb.com/

David

Re: Making linkat() able to overwrite the target

[Omar Sandoval]

On Fri, Jan 17, 2020 at 11:42:55AM +0000, David Howells wrote:
> Hi Omar,
> 
> Do you still have your AT_REPLACE patches?  You said that you'd post a v4
> series, though I don't see it.  I could make use of such a feature in
> cachefiles inside the kernel.  For my original question, see:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter
> 
> And do you have ext4 support for it?

Hi,

Yes I still have those patches lying around and I'd be happy to dust
them off and resend them. I don't have ext4 support. I'd be willing to
take a stab at ext4 once Al is happy with the VFS part unless someone
more familiar with ext4 wants to contribute that support.

Thanks for reviving interesting in this!

Re: Making linkat() able to overwrite the target

[David Howells]

Omar Sandoval <osandov@osandov.com> wrote:

> Yes I still have those patches lying around and I'd be happy to dust
> them off and resend them.

That would be great if you could.  I could use them here:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter

I'm performing invalidation by creating a vfs_tmpfile() and then replacing the
on-disk file whilst letting ops resume on the temporary file.  Replacing the
on-disk file currently, however, involves unlinking the old one before I can
link in a new one - which leaves a window in which nothing is there.  I could
use one or more side dirs in which to create new files and rename them over,
but that has potential lock bottleneck issues - and is particularly fun if an
entire volume is invalidated (e.g. AFS vos release).

David

Re: Making linkat() able to overwrite the target

[Christoph Hellwig]

On Tue, Jan 14, 2020 at 04:34:25PM +0000, David Howells wrote:
> 
> when a file gets invalidated by the server - and, under some circumstances,
> modified locally - I have the cache create a temporary file with vfs_tmpfile()
> that I'd like to just link into place over the old one - but I can't because
> vfs_link() doesn't allow you to do that.  Instead I have to either unlink the
> old one and then link the new one in or create it elsewhere and rename across.
> 
> Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE, that
> causes the target to be replaced and not give EEXIST?  Or make it so that
> rename() can take a tmpfile as the source and replace the target with that.  I
> presume that, either way, this would require journal changes on ext4, xfs and
> btrfs.

This sounds like a very useful primitive, and from the low-level XFS
point of view should be very easy to implement and will not require any
on-disk changes.  I can't really think of any good userspace interface but
a new syscall, though.