Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode eviction path

Jason Gunthorpe
2022-09-30T18:06:54Z

On Fri, Sep 30, 2022 at 10:56:27AM -0700, Dan Williams wrote:
> Jan Kara wrote:
> [..]
> > I agree this is doable but there's the nasty sideeffect that inode reclaim
> > may block for abitrary time waiting for page pinning. If the application
> > that has pinned the page requires __GFP_FS memory allocation to get to a
> > point where it releases the page, we even have a deadlock possibility.
> > So it's better than the UAF issue but still not ideal.
>
> I expect VMA pinning would have similar deadlock exposure if pinning a
> VMA keeps the inode allocated. Anything that puts a page-pin release
> dependency in the inode freeing path can potentially deadlock a reclaim
> event that depends on that inode being freed.

I think the desire would be to go from the VMA to an inode_get and
hold the inode reference for the from the pin_user_pages() to the
unpin_user_page(), ie prevent it from being freed in the first place.

It is a fine idea, the trouble is just the high complexity to get
there.

However, I wonder if the trucate/hole punch paths have the same
deadlock problem?

I agree with you though, given the limited options we should convert
the UAF into an unlikely deadlock.

Jason