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Date:   Thu, 22 Sep 2022 17:41:08 -0700
From:   Dan Williams <>
To:     Dave Chinner <>,
        Jason Gunthorpe <>
CC:     Dan Williams <>,
        <>, Matthew Wilcox <>,
        Jan Kara <>, "Darrick J. Wong" <>,
        Christoph Hellwig <>,
        John Hubbard <>,
        <>, <>,
        <>, <>,
Subject: Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode
 eviction path

Dave Chinner wrote:
> On Wed, Sep 21, 2022 at 07:28:51PM -0300, Jason Gunthorpe wrote:
> > On Thu, Sep 22, 2022 at 08:14:16AM +1000, Dave Chinner wrote:
> > 
> > > Where are these DAX page pins that don't require the pin holder to
> > > also hold active references to the filesystem objects coming from?
> > 
> > O_DIRECT and things like it.
> O_DIRECT IO to a file holds a reference to a struct file which holds
> an active reference to the struct inode. Hence you can't reclaim an
> inode while an O_DIRECT IO is in progress to it. 
> Similarly, file-backed pages pinned from user vmas have the inode
> pinned by the VMA having a reference to the struct file passed to
> them when they are instantiated. Hence anything using mmap() to pin
> file-backed pages (i.e. applications using FSDAX access from
> userspace) should also have a reference to the inode that prevents
> the inode from being reclaimed.
> So I'm at a loss to understand what "things like it" might actually
> mean. Can you actually describe a situation where we actually permit
> (even temporarily) these use-after-free scenarios?

Jason mentioned a scenario here:

Multi-thread process where thread1 does open(O_DIRECT)+mmap()+read() and
thread2 does memunmap()+close() while the read() is inflight.

Sounds plausible to me, but I have not tried to trigger it with a focus

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