From: Hugh Dickins <hughd@google.com>
To: Snaipe <snaipe@arista.com>
Cc: hughd@google.com, ovt@google.com, corbet@lwn.net,
akpm@linux-foundation.org, brauner@kernel.org,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org
Subject: Re: [PATCH] shmem: add support for user extended attributes
Date: Mon, 14 Aug 2023 20:51:59 -0700 (PDT) [thread overview]
Message-ID: <986c412c-669a-43fe-d72a-9e81bca8211@google.com> (raw)
In-Reply-To: <20230814082339.2006418-1-snaipe@arista.com>
On Mon, 14 Aug 2023, Snaipe wrote:
> > Your sending this patch does help to raise the priority for my
> > sending that patch: thank you; but I cannot promise when that will be.
>
> Hey Hugh,
>
> Just as an additional data point, if it helps with priority :)
>
> The lack of support of user xattrs on tmpfs our last remaining blocker for
> using unprivileged overlayfs mounts that use a tmpfs for the upper dir & work
> dir. Not that it isn't possible to use unprivileged overlayfs mounts in general,
> but not having this option means that use-cases for discardable upper layer
> changes are harder to clean up correctly when not on an tmpfs mount whose
> lifetime is bound to a mount namespace.
>
> I don't think there's any rush; we can live with rmdir failing with EIO for now,
> but it'd be great to see this fixed rather than having to implement expensive
> cleanup routines that have to remove the upper+work dirs recursively with
> CAP_DAC_OVERRIDE.
Thanks for the encouragement. At the time that I wrote that (20 July)
I did not expect to get around to it for months. But there happens to
have been various VFS-involving works going on in mm/shmem.c recently,
targeting v6.6: which caused me to rearrange priorities, and join the
party with tmpfs user xattrs, see
https://lore.kernel.org/linux-fsdevel/e92a4d33-f97-7c84-95ad-4fed8e84608c@google.com/
Which Christian Brauner quickly put into his vfs.git (vfs.tmpfs branch):
so unless something goes horribly wrong, you can expect them in v6.6.
There's a lot that you wrote above which I have no understanding of
whatsoever (why would user xattrs stop rmdir failing?? it's okay, don't
try to educate me, I don't need to know, I'm just glad if they help you).
Though your mention of "unprivileged" does make me shiver a little:
Christian will understand the implications when I do not, but I wonder
if my effort to limit the memory usage of user xattrs to "inode space"
can be be undermined by unprivileged mounts with unlimited (or default,
that's bad enough) nr_inodes.
If so, that won't endanger the tmpfs user xattrs implementation, since
the problem would already go beyond those: can an unprivileged mount of
tmpfs allow its user to gobble up much more memory than is good for the
rest of the system?
Hugh
next prev parent reply other threads:[~2023-08-15 3:52 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-20 6:54 [PATCH] shmem: add support for user extended attributes Oleksandr Tymoshenko
2023-07-20 16:56 ` Hugh Dickins
2023-07-20 17:09 ` Oleksandr Tymoshenko
2023-07-20 17:42 ` Hugh Dickins
2023-08-14 8:23 ` Snaipe
2023-08-15 3:51 ` Hugh Dickins [this message]
2023-08-15 7:46 ` Franklin “Snaipe” Mathieu
2023-08-15 8:23 ` Christian Brauner
2023-08-21 17:52 ` Hugh Dickins
2023-08-22 9:01 ` Christian Brauner
2023-08-22 17:50 ` Oleksandr Tymoshenko
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