linux-ext4 - Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode eviction path

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Date:   Fri, 30 Sep 2022 15:06:47 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        akpm@...ux-foundation.org, Matthew Wilcox <willy@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode
 eviction path

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