On Fri, 8 Dec 2023 11:14:32 +1100, [email protected] wrote:
> On Tue, Dec 05, 2023 at 07:38:33PM +0800, [email protected] wrote:
> > Hi, all
> >
> > I would like to ask if the conflict between xfs inode recycle and vfs
> > rcu-walk
> > which can lead to null pointer references has been resolved?
> >
> > I browsed through emails about the following patches and their discussions:
> > -
> > https://lore.kernel.org/linux-xfs/[email protected]/
> > -
> > https://lore.kernel.org/linux-xfs/[email protected]/
> > -
> > https://lore.kernel.org/linux-xfs/[email protected]/
> >
> > And then came to the conclusion that this problem has not been solved, am I
> > right? Did I miss some patch that could solve this problem?
>
> We fixed the known problems this caused by turning off the VFS
> functionality that the rcu pathwalks kept tripping over. See commit
> 7b7820b83f23 ("xfs: don't expose internal symlink metadata buffers to
> the vfs").
Sorry for the delay.
The problem I encountered in the production environment was that during the
rcu walk process the ->get_link() pointer was NULL, which caused a crash.
As far as I know, commit 7b7820b83f23 ("xfs: don't expose internal symlink
metadata buffers to the vfs") first appeared in:
- https://lore.kernel.org/linux-fsdevel/YZvvP9RFXi3%2FjX0q@bfoster/
Does this commit solve the problem of NULL ->get_link()? And how?
>
> Apart from that issue, I'm not aware of any other issues that the
> XFS inode recycling directly exposes.
>
> > According to my understanding, the essence of this problem is that XFS
> > reuses
> > the inode evicted by VFS, but VFS rcu-walk assumes that this will not
> > happen.
>
> It assumes that the inode will not change identity during the RCU
> grace period after the inode has been evicted from cache. We can
> safely reinstantiate an evicted inode without waiting for an RCU
> grace period as long as it is the same inode with the same content
> and same state.
>
> Problems *may* arise when we unlink the inode, then evict it, then a
> new file is created and the old slab cache memory address is used
> for the new inode. I describe the issue here:
>
> https://lore.kernel.org/linux-xfs/[email protected]/
And judging from the relevant emails, the main reason why ->get_link() is set
to NULL should be the lack of synchronize_rcu() before xfs_reinit_inode() when
the inode is chosen to be reused.
However, perhaps due to performance reasons, this solution has not been merged
for a long time. How is it now?
Maybe I am missing something in the threads of mail?
Thank you very much. :)
Jinliang Zheng
>
> That said, we have exactly zero evidence that this is actually a
> problem in production systems. We did get systems tripping over the
> symlink issue, but there's no evidence that the
> unlink->close->open(O_CREAT) issues are manifesting in the wild and
> hence there hasn't been any particular urgency to address it.
>
> > Are there any recommended workarounds until an elegant and efficient
> > solution
> > can be proposed? After all, causing a crash is extremely unacceptable in a
> > production environment.
>
> What crashes are you seeing in your production environment?
>
> -Dave.
> --
> Dave Chinner
> [email protected]
