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

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Date:   Mon, 3 Oct 2022 09:55:03 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, 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 30-09-22 15:06:47, Jason Gunthorpe wrote:
> 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.

Yes, that was the idea how to avoid UAF problems.

> 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?

Do you mean someone requiring say truncate(2) to complete on file F in
order to unpin pages of F? That is certainly a deadlock but it has always
worked this way for DAX so at least applications knowingly targetted at DAX
will quickly notice and avoid such unwise dependency ;).

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR